wl1271_cmd.c: cleanup char => u8

This is just a clean up and doesn't make a functional difference.  It keeps the
lint checkers happy.

Signed-off-by: Dan Carpenter <error27@gmail.com>
Reviewed-by: Kalle Valo <kalle.valo@nokia.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>