firmware: Reword error messages when kernel is unable to find firmware

The current error message printed when kernel fails to find the firmware
suggests that the entire operation has failed when the user helper may
still find the firmware. As we pass the return code back up, let the
calling driver determine if the entire operation has failed or not.

Change-Id: I5d8ce65e533e1925e98a49a0a88280549af75cb9
Signed-off-by: Kyle Yan <kyan@codeaurora.org>