mwifiex: remove an unneeded NULL check

We dereference adapter in the error handling code so this needed to
be fixed.  This function is always called like:
	adapter->if_ops.host_to_card(adapter, ...);
so adapter can never be NULL and I've removed the NULL check.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>