arm64: rockchip: rk_pinctrl: Fix clear bits in SYSCON_MODIFY

r351187 change the SYSCON_WRITE to SYSCON_MODIFY but didn't changed the
mask variable that used to hold the bitmask in the upper 16 bits of the
register that control which bits are changed. So we ended up clearing
bit from the upper 16bits half which are always 0 after a read.
Use the correct bit mask for bits that we want to clear.

MFC after: 3 days