staging: mt7621-pinctrl: replace uint8_t type with u8 for 'gpio' field

Struct 'rt2880_priv' contains a field 'gpio' which is defined
as uint8_t and should be defined with 'u8' which is preferred.
Update some cast along the code related with this new change.

Signed-off-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>