ec/google/chromeec: Simplify KEYBOARD_BACKLIGHT error handling

Simplify the implementation of setting the keyboard backlight PWM
value. Host command stubs typcially don't need to examine the host
command's return value as stored in cmd_code because that level of
detail is not very interesting. Higher value error codes are returned in
actual result structures.

This host command can return EC_RES_ERROR for out of range PWM values
which is already a generic error and unlikely to happen since we already
limit the range to 0..100 here. Finally, none of the callers in coreboot
check the return value.

BUG=b:258126464
BRANCH=none
TEST=none

Change-Id: If17bc4e31baba02ba2f7ae8e7a5cbec7f97688c5
Signed-off-by: Caveh Jalali <caveh@chromium.org>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/69369
Reviewed-by: Daisuke Nojiri <dnojiri@chromium.org>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/ec/google/chromeec/ec.c

@@ -87,10 +87,10 @@ int google_chromeec_kbbacklight(int percent)
 		.cmd_dev_index = 0,
 	};
 
-	google_chromeec_command(&cmd);
-	printk(BIOS_DEBUG, "Google Chrome set keyboard backlight: status (%x)\n",
-	       cmd.cmd_code);
-	return cmd.cmd_code;
+	if (google_chromeec_command(&cmd) != 0)
+		return -1;
+
+	return 0;
 }
 
 void google_chromeec_post(uint8_t postcode)