PCI: mt7621: Convert driver into 'bool'

The driver is not ready yet to be compiled as a module since it depends
on some symbols not exported on MIPS.  We have the following current
problems:

  Building mips:allmodconfig ... failed
  --------------
  Error log:
  ERROR: modpost: missing MODULE_LICENSE() in drivers/pci/controller/pcie-mt7621.o
  ERROR: modpost: "mips_cm_unlock_other" [drivers/pci/controller/pcie-mt7621.ko] undefined!
  ERROR: modpost: "mips_cpc_base" [drivers/pci/controller/pcie-mt7621.ko] undefined!
  ERROR: modpost: "mips_cm_lock_other" [drivers/pci/controller/pcie-mt7621.ko] undefined!
  ERROR: modpost: "mips_cm_is64" [drivers/pci/controller/pcie-mt7621.ko] undefined!
  ERROR: modpost: "mips_gcr_base" [drivers/pci/controller/pcie-mt7621.ko] undefined!

Temporarily move from 'tristate' to 'bool' until a better solution is
ready.

Also RALINK is redundant because SOC_MT7621 already depends on it.
Hence, simplify condition.

Fixes: 2bdd5238 ("PCI: mt7621: Add MediaTek MT7621 PCIe host controller driver").
Signed-off-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Reviewed-and-Tested-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>