mwifiex: fix coding style issue in mwifiex_deauthenticate

Documentation/CodingStyle says
"Do not unnecessarily use braces where a single statement will do."

Use "switch/case", instead of "if/else_if/else", so that more cases
can be added later.

Signed-off-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: Avinash Patil <patila@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>