mwifiex: code rearrangement for better readability

Use negative check for 'status' and return from the function.
This improves readability by avoiding line splits. Also, local
variable is used for start window calculations.

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