x86: microcode_amd: fix compile warning

Impact: fix build warning

  CC      arch/x86/kernel/microcode_amd.o
arch/x86/kernel/microcode_amd.c: In function ‘request_microcode_fw’:
arch/x86/kernel/microcode_amd.c:393: warning: passing argument 2 of ‘generic_load_microcode’ discards qualifiers from pointer target type

(Respect "const" qualifier of firmware->data.)

Signed-off-by: Andreas Herrmann <andreas.herrmann3@amd.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>