mwifiex: fix comparison expression

To avoid underflows signed variables should be used in expression.

The problem has been detected using proposed semantic patch
scripts/coccinelle/tests/unsigned_lesser_than_zero.cocci [1].

[1]: http://permalink.gmane.org/gmane.linux.kernel/2038576

Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Acked-by: Amitkumar Karwar <akarwar@marvell.com>
Signed-off-by: David S. Miller <davem@davemloft.net>