staging: mt7621-pinctrl: use ternary operator return in rt2880_get_group_name

Trivial change to have only one return in 'rt2880_get_group_name'
function using a ternary operator instead of an if statement and
two returns.

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