octeontx2-af: Remove set but not used variable 'dev'

Fixes gcc '-Wunused-but-set-variable' warning:

drivers/net/ethernet/marvell/octeontx2/af/cgx.c: In function 'cgx_fwi_event_handler':
drivers/net/ethernet/marvell/octeontx2/af/cgx.c:257:17: warning:
 variable 'dev' set but not used [-Wunused-but-set-variable]

It never be used since introduction in
commit 1463f382 ("octeontx2-af: Add support for CGX link management")

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>