The mask/shift computation used the pin group number instead of the pin
number, resulting in always modifying group 4 when applying muxes, so
fix it to consistently use the pin number.
Fixes: 0755c2d117 ("brcm63xx: add pinctrl support")
Reported-by: Daniel Gonzalez Cabanelas <dgcbueu@gmail.com>
Signed-off-by: Jonas Gorski <jonas.gorski@gmail.com>