[media] radio-tea5777: Add support for tuning AM

This completes my work on the Griffin radioSHARK2 driver, let me use this
opportunity to thank Hisaaki Shibata for his generous donation of a
Griffin radioSHARK2 to me, which has made this driver possible.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>