sfc: remove set but not used variable 'nic_data'

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

drivers/net/ethernet/sfc/mcdi_functions.c: In function 'efx_mcdi_ev_init':
drivers/net/ethernet/sfc/mcdi_functions.c:79:28: warning:
 variable 'nic_data' set but not used [-Wunused-but-set-variable]

commit 4438b587 ("sfc: move MCDI event queue management code")
introduces this unused variable.

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