net: wireless: rtlwifi: Do not always include drivers in obj-m

In four of the rtlwifi drivers, the Makefile contains superfluous
statements indicating the compilation of the driver as an LKM
regardless of the corresponding Kconfig option.

If the corresponding option is set to 'y', the build system will then
see the object file in obj-m and obj-y, which leads to a compilation
as a built-in only. Even though this leads to the desired behavior,
the unconditional appearance in obj-m is confusing for someone reading
the Makefile.

This patch removes the superfluous Makefile statements.

Signed-off-by: Andreas Ruprecht <rupran@einserver.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>