gprs_llc: remove the FCS from the msgb by trim'ing the msgb

After checking the FCS, it's no use. The FCS should also not
appear on `hexdump(msgb_l3(MSG), msgb_l3len(MSG))`.

Change-Id: I27e061ead86395a336b67c7aead93d305a0f2ae8
diff --git a/src/gprs/gprs_sndcp.c b/src/gprs/gprs_sndcp.c
index 77f8ca6..52eeb75 100644
--- a/src/gprs/gprs_sndcp.c
+++ b/src/gprs/gprs_sndcp.c
@@ -792,7 +792,7 @@
 
 	npdu_num = (suh->npdu_high << 8) | suh->npdu_low;
 	npdu = (uint8_t *)suh + sizeof(*suh);
-	npdu_len = (msg->data + msg->len) - npdu - 3;	/* -3 'removes' the FCS */
+	npdu_len = (msg->data + msg->len) - npdu;
 
 	if (npdu_len <= 0) {
 		LOGP(DSNDCP, LOGL_ERROR, "Short SNDCP N-PDU: %d\n", npdu_len);