mwifiex: remove unneeded NULL check

We dereference "rate" on the lines before so the checks here are too
late to help.  This function is only called from
mwifiex_dump_station_info() and "rate" is always a non-NULL pointer
so the check can be removed.

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>