All the mailbox messages sent to AF needs to be guarded
by mutex lock. Add the missing lock in otx2_get_pauseparam
function.

Fixes: 75f36270 ("octeontx2-pf: Support to enable/disable pause frames via ethtool")
Signed-off-by: Subbaraya Sundeep <sbhatta@marvell.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>