utils: usrp_burn_mb_eeprom can now burn multiple EEPROM values

* example: usrp_burn_mb_eeprom --key="name,ip-addr" --val="my_usrp,192.168.10.3"
* As before, omitting the --val argument will just read off all named keys
* If number of given keys and vals don't match, the util will throw an error
usrp_burn_mb_eeprom: removed unnecessary if-statement and fixed whitespace issue