mwifiex: correct sleep delay counter

Maximum delay for waking up card is 50 ms. Because of typo in
counter, this delay goes to 500ms. This patch fixes the bug.

Cc: <stable@vger.kernel.org> # 3.2+
Signed-off-by: Avinash Patil <patila@marvell.com>
Signed-off-by: Amitkumar Karwar <akarwar@marvell.com>
Signed-off-by: Yogesh Ashok Powar <yogeshp@marvell.com>
Signed-off-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>