drivers: modem: sara-r4: fix modem reset

This fixes two problems in modem_reset():

- mdata.net_iface is used without checking that it is valid.
  Now, we validate mdata_net_iface before usage, and give a warning
  if it was not set.

- if the modem does not perform network registration within the
  given time of 20s, in the previous implementation the driver
  gave up.
  Now, the driver disables RF for a second, enables it again and
  gives the modem another 20s to register. This has been seen
  to help in roaming situations.

Tested on Sara R4.

Signed-off-by: Hans Wilmers <hans@wilmers.no>