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