cpu/x86/smi_trigger: use enum cb_err as apm_control return type

Even though the return value from apm_control isn't checked at any of
its call sites, using the cb_err enum instead of an integer as return
type makes it clearer what the returned value means.

Signed-off-by: Felix Held <felix-coreboot@felixheld.de>
Change-Id: I07ced74cae915df52a9d439835b84237d51fdd11
Reviewed-on: https://review.coreboot.org/c/coreboot/+/79835
Reviewed-by: Jérémy Compostella <jeremy.compostella@intel.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Eric Lai <ericllai@google.com>
Reviewed-by: Matt DeVillier <matt.devillier@amd.corp-partner.google.com>

src/include/cpu/x86/smm.h

@@ -43,7 +43,7 @@
 #endif
 
 /* Send cmd to APM_CNT with HAVE_SMI_HANDLER checking. */
-int apm_control(u8 cmd);
+enum cb_err apm_control(u8 cmd);
 u8 apm_get_apmc(void);
 
 void io_trap_handler(int smif);