staging: mt7621-pinctrl: Test devm_kzalloc for failure while improving the code

This patch tests the return of devm_kzalloc for failure and
improves the code.

Signed-off-by: Nishad Kamdar <nishadkamdar@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>