staging: vchiq_arm: handle error case of get_user_pages

It's possible that get_user_pages() could fail. So evaluate its
return code and handle this error case properly.

This issue has been found by Cppcheck.

Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>