firmware loader: let direct loading back on 'firmware_buf'

Firstly 'firmware_buf' is introduced to make all loading requests
to share one firmware kernel buffer, so firmware_buf should
be used in direct loading for saving memory and speedup firmware
loading.

Secondly, the commit below

	abb139e7(firmware:teach
	the kernel to load firmware files directly from the filesystem)

introduces direct loading for fixing udev regression, but it
bypasses the firmware cache meachnism, so this patch enables
caching firmware for direct loading case since it is still needed
to solve drivers' dependency during system resume.

Cc: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Ming Lei <ming.lei@canonical.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>