drivers/net: fix FW version query

drivers/net: fix FW version query

Fixes a few different things:
* Remove 'fw_version' NULL checks, it is allowed if the 'fw_size' is
  zero, 'fw_version' being NULL but 'fw_size' not zero condition checked
  in ethdev layer
* Be sure required buffer size is returned if provided one is not big
  enough, instead of returning success (0)
* Document in doxygen comment the '-EINVAL' is a valid return type
* Take into account that 'snprintf' can return negative value
* Cast length to 'size_t' to compare it with 'fw_size'

Fixes: bb42aa9ffe4e ("net/atlantic: configure device start/stop")
Fixes: ff70acdf4299 ("net/axgbe: support reading FW version")
Fixes: e2652b0a20a0 ("net/bnxt: support get FW version")
Fixes: cf0fab1d2ca5 ("net/dpaa: support firmware version get API")
Fixes: 748eccb97cdc ("net/dpaa2: add support for firmware version get")
Fixes: b883c0644a24 ("net/e1000: add firmware version get")
Fixes: 293430677e9c ("net/enic: add handler to return firmware version")
Fixes: 1f5ca0b460cd ("net/hns3: support some device operations")
Fixes: bd5b86732bc7 ("net/hns3: modify format for firmware version")
Fixes: ed0dfdd0e976 ("net/i40e: add firmware version get")
Fixes: e31cb9a36298 ("net/ice: support FW version getting")
Fixes: 4f09bc55ac3d ("net/igc: implement device base operations")
Fixes: eec10fb0ce6b ("net/ionic: support FW version")
Fixes: 8b0b56574269 ("net/ixgbe: add firmware version get")
Fixes: 4d9f5b8adc02 ("net/octeontx2: add FW version get operation")
Fixes: f97b56f9f12e ("net/qede: support FW version query")
Fixes: 83fef46a22b2 ("net/sfc: add callback to retrieve FW version")
Fixes: bc84ac0fadef ("net/txgbe: support getting FW version")
Fixes: 21913471202f ("ethdev: add firmware version get")
Cc: stable@dpdk.org
Signed-off-by: Ferruh Yigit <ferruh.yigit@intel.com>
Acked-by: Ajit Khaparde <ajit.khaparde@broadcom.com>
Acked-by: Andrew Rybchenko <andrew.rybchenko@oktetlabs.ru>
Acked-by: Haiyue Wang <haiyue.wang@intel.com>
Acked-by: Rasesh Mody <rmody@marvell.com>
Acked-by: Jiawen Wu <jiawenwu@trustnetic.com>
Acked-by: Beilei Xing <beilei.xing@intel.com>
Acked-by: Hemant Agrawal <hemant.agrawal@nxp.com>