media: dvb: check for ion mapping errors

Mapping a linear buffer allocated using ion to kernel space might
result in an error other than NULL pointer. Current code only checked
for NULL so errors can go undetected as a valid buffer mapping.

Change-Id: I3d0af279b6ac0d6cc1689ba910ebb5c9d4a0a872
Signed-off-by: Gilad Broner <gbroner@codeaurora.org>