rt2x00: rt2x00pci: fix build error on Ralink RT3x5x SoCs

The rt2800pci driver supports the built-in wireless
MAC of the Ralink RT3x5x SoCs. However building the
driver for these SoCs leads to the following error:

    LD      init/built-in.o
  drivers/built-in.o: In function `rt2800pci_rxdone_tasklet':
  <...>/drivers/net/wireless/rt2x00/rt2800pci.c:1012: undefined reference to `rt2x00pci_rxdone'
  drivers/built-in.o:(.rodata+0x4780): undefined reference to `rt2x00pci_initialize'
  drivers/built-in.o:(.rodata+0x4784): undefined reference to `rt2x00pci_uninitialize'
  drivers/built-in.o:(.rodata+0x47bc): undefined reference to `rt2x00pci_flush_queue'
  drivers/built-in.o:(.rodata+0x4818): undefined reference to `rt2x00pci_regbusy_read'
  make[5]: *** [vmlinux] Error 1

The missing functions are provided by the rt2x00pci
module. This module is only selected by the rt2800pci
driver if PCI support is enabled in the kernel, because
some parts of the rt2x00pci code depends on PCI support.

PCI support is not available on the RT3x5x SoCs because
those have no PCI host controller at all.

Move the non PCI specific code from rt2x00pci into a
separate module. This makes it possible to use that
code even if PCI support is disabled. The affected
functions are used by all of the rt2x00 PCI drivers
so select the new module for those drivers.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>