mwifiex: code rearrangement in mwifiex_get_rd_port()

Get rid of 'if else' usage by returning in 'if' block.
This improves readability by removing indentations.

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>