net/sfc: fix minimum number of Rx descriptors in ESSB mode

net/sfc: fix minimum number of Rx descriptors in ESSB mode

Number of descriptors in equal stride super-buffer Rx mode defines
number of packet buffers to be used. Each HW Rx descriptor has
many packet buffers and the number depends on total size of mbuf
and CONFIG_RTE_DRIVER_MEMPOOL_BUCKET_SIZE_KB value.
Typically it makes a bit less than 32 buffers per descriptor.
Since HW Rx descriptors must be pushed by 8, it makes about 256
as required minimum. Double it in advertised minimum to allow for
at least 2 refill blocks.

Fixes: 390f9b8d82c9 ("net/sfc: support equal stride super-buffer Rx mode")

Signed-off-by: Andrew Rybchenko <arybchenko@solarflare.com>