alx: fix missing unlock on error in alx_set_pauseparam()

Add the missing unlock before return from function alx_set_pauseparam()
in the error handling case.

Fixes: 4a5fe57e ("alx: use fine-grained locking instead of RTNL")
Signed-off-by: Pu Lehui <pulehui@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>