staging: mt7621-pci: use builtin_platform_driver()

Macro builtin_platform_driver can be used for builtin drivers
that don't do anything in driver init. So, use the macro
builtin_platform_driver and remove some boilerplate code.

Signed-off-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Link: https://lore.kernel.org/r/20200321133624.31388-1-sergio.paracuellos@gmail.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>