app/testpmd: fix bitmap of link speeds when force speed

app/testpmd: fix bitmap of link speeds when force speed

Currently, when the user sets force link speed through 'link_speeds',
bit(0) of 'link_speeds' is not set to 1(ETH_LINK_SPEED_FIXED),
which conflicts with the definition.

Fixes: 88fbedcd5e5a ("app/testpmd: move speed and duplex parsing in a function")
Cc: stable@dpdk.org
Signed-off-by: Huisong Li <lihuisong@huawei.com>
Signed-off-by: Min Hu (Connor) <humin29@huawei.com>
Acked-by: Ferruh Yigit <ferruh.yigit@intel.com>
Acked-by: Ajit Khaparde <ajit.khaparde@broadcom.com>