Lattice ECP3 FPGA: Check firmware pointer

This patch corrects a lack of testing.
If fw is NULL when calling firmware_load(), it results in a kernel oops.

Signed-off-by: Jean-Michel Hautbois <jean-michel.hautbois@vodalys.com>
Reviewed-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>