staging: mt7621-pci: make use of kernel clock apis

staging: mt7621-pci: make use of kernel clock apis

MT7621 SoC clock driver has already mainlined in
'commit 48df7a26f470 ("clk: ralink: add clock driver for mt7621 SoC")'
Hence, we can make use of kernel clock apis and avoid to
directly set bits in clock gate related registers for pci.

Signed-off-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Link: https://lore.kernel.org/r/20210505121736.6459-2-sergio.paracuellos@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>