staging: android: ion: Check return value of ion_buffer_kmap_get

GCC warns that vaddr is set but unused. Check the return value of
ion_buffer_kmap_get to make vaddr useful and make sure everything
is properly configured before beginning a DMA.

Suggested-by: Laura Abbott <labbott@redhat.com>
Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>