Quectel EC21 does not understand AT+CPSB
The Quectel EC21 modem does not understand the AT+CPSB command, and we did not find a suitable replacement in the Quectel_EC25&EC21_AT_Commands_Manual_V1.3.pdf AT+CPSB gives an error on this modem, so we just skip it.
https://git.sysmocom.de/sysmocom/ofono/commit/288f263ab681b4e5dbdddd85d4093e1874e773c4