sfc: add missing annotation for efx_ef10_try_update_nic_stats_vf()

Sparse reports a warning at efx_ef10_try_update_nic_stats_vf()
warning: context imbalance in efx_ef10_try_update_nic_stats_vf()
	- unexpected unlock
The root cause is the missing annotation at
efx_ef10_try_update_nic_stats_vf()
Add the missing _must_hold(&efx->stats_lock) annotation

Signed-off-by: Jules Irenge <jbi.octave@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>