V4L/DVB (7092): radio-sf16fmr2: fix request_region() validation [bugzilla 9699]

This patch changed the request_region() validation to avoid invalid return.
Thanks to Roland Kletzing <devzero@web.de> for bug report and data collection.

Signed-off-by: Douglas Schilling Landgraf <dougsland@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>