firmware loader: fix another compile warning with PM_SLEEP unset

This patch fixes another compiling warning with PM_SLEEP unset:

drivers/base/firmware_class.c:221:29: warning: 'fw_lookup_buf' defined
but not used [-Wunused-function]

This time I do build kernel with both PM_SLEEP set and unset, and no
warning found any more with the patch.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Ming Lei <ming.lei@canonical.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>