Deal with Red Hat backporting request_firmware_nowait change.

Deal with Red Hat backporting request_firmware_nowait change.

An extra 'gfp' parameter was added to request_firmware_nowait() in the
vanilla 2.6.33 kernel and the current <linux/firmware.h> compatibility
header checks for this kernel version.  Unfortunately, Red Hat
backported the change to their 2.6.32 kernel so the code in our
compatibility header breaks on those kernels.

Add a simple configure test to see if the gfp parameter is required and
use this to define CONFIG_COMEDI_REQUEST_FIRMWARE_NOWAIT_HAS_GFP if it
is required.  Change out <linux/firmware.h> compatibility header to
check this macro instead of checking the kernel version directly.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>