staging: mt7621-pinctrl: init *map to NULL for correct memory assignation

pinctrl_utils_reserve_map() calls krealloc() on *map. Because of this
*map need to be initialized to NULL before calling it.

Fixes: 62b6215c ("staging: mt7621-pinctrl: make use of pinctrl_utils_reserve_map")
Signed-off-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>