Since the modem states patch the delay for power-on wasn't honored
for Sierra devices. Fix that using the new power-on-done handler
and also fix the bug where if the modem was removed, the plugin
would crash because it wasn't handling the timeout removal.
Also remove the explicit PIN check since that's now handled by the
generic GSM code before the modem is even exported over DBus.
For QCDM devices we want most of what MMSerialPort does, but not
the AT command handling stuff since the commands and responses
aren't AT commands nor are they even strings. So convert everything
that MMSerialPort does into a GByteArray, and let MMAtSerialPort
handle the conversion to strings when necessary.
Dan Williams