greybus: firmware: s/should/must

Replace 'should' with 'must' to clear the expectation a bit more.

Reported-by: Alex Elder <[e-mail]>
Signed-off-by: Viresh Kumar <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/staging/greybus/fw-download.c

···
 	request = op->request->payload;
 	tag = (const char *)request->firmware_tag;
 
-	/* firmware_tag should be null-terminated */
+	/* firmware_tag must be null-terminated */
 	if (strnlen(tag, GB_FIRMWARE_TAG_MAX_SIZE) == GB_FIRMWARE_TAG_MAX_SIZE) {
 		dev_err(fw_download->parent,
 			"firmware-tag is not null-terminated\n");