transport/AT: Make sure PDU has upper case hex digits

Some modems may reject AT+CSIM if PDU contains lower
case hex digits [a-f]. Modem response is "ERROR"
without any error code.

This patch converts each PDU to upper case.

Tested with Sierra Wireless EM7565.
Example:
AT+CSIM=14,"00a40004023F00"
ERROR
AT+CSIM=14,"00A40004023F00"
+CSIM: 4,"612F"

OK


Change-Id: I318e36abc7ae975c62d32b7fe0ec949bf5997d13

pySim/transport/modem_atcmd.py

@@ -99,6 +99,9 @@ class ModemATCommandLink(LinkBase):
 		pass # Nothing to do really ...
 
 	def _send_apdu_raw(self, pdu):
+		# Make sure pdu has upper case hex digits [A-F]
+		pdu = pdu.upper()
+
 		# Prepare the command as described in 8.17
 		cmd = 'AT+CSIM=%d,\"%s\"' % (len(pdu), pdu)