rtlwifi: Fix build errors for unusual cases

The present build configuration for the rtlwifi family of drivers will
fail under two known conditions:

(1) If rtlwifi is selected without selecting any of the dependent drivers,
    there are errors in the build.
(2) If the PCI drivers are built into the kernel and the USB drivers are modules,
    or vice versa, there are missing globals.

The first condition is fixed by never building rtlwifi unless at least one
of the device drivers is selected. The second failure is fixed by splitting
the PCI and USB codes out of rtlwifi, and creating their own mini drivers.
If the drivers that use them are modules, they will also be modules.

Although a number of files are touched by this patch, only Makefile and Kconfig
have undergone significant changes. The only modifications to the other files
were to export entry points needed by the new rtl_pci and rtl_usb units, or to
rename two variables that had names that were likely to cause namespace collisions.

Reported-by: Fengguang Wu <fengguang.wu@intel.com>  [Condition 1]
Reported-by: Ben Hutchings <bhutchings@solarflare.com> [Condition 2]
Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
Cc: Ben Hutchings <bhutchings@solarflare.com>
Cc: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>