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: 2bdd5238e756 ("PCI: mt7621: Add MediaTek MT7621 PCIe host controller driver").
Signed-off-by: Sergio Paracuellos <[e-mail]>
Reviewed-and-Tested-by: Guenter Roeck <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

drivers/pci/controller/Kconfig

···
 	  If unsure, say Y if you have an Apple Silicon system.
 
 config PCIE_MT7621
-	tristate "MediaTek MT7621 PCIe Controller"
-	depends on (RALINK && SOC_MT7621) || (MIPS && COMPILE_TEST)
+	bool "MediaTek MT7621 PCIe Controller"
+	depends on SOC_MT7621 || (MIPS && COMPILE_TEST)
 	select PHY_MT7621_PCI
 	default SOC_MT7621
 	help