mwifiex: do not overwrite error code from lower layer driver

Instead of converting it to a bogus error code -1, we should
return the original error code from lower layer driver. This
error code will be printed so it may give user some clues on
what has happened.

Signed-off-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>