6lowpan: iphc: Fix missing braces for if statement

CHECK: braces {} should be used on all arms of this statement
+	if ((iphc0 & 0x03) != LOWPAN_IPHC_TTL_I)
[...]
+	else {
[...]

Signed-off-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>