Staging: rtl8188eu: Replace if-else block with switch-case

Replace if-else block with switch-case to make it more compact
and increase readability.

Signed-off-by: Vatika Harlalka <vatikaharlalka@gmail.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>