staging: dgnc: clean up the dgnc_get_modem_info()

the "ch" in dgnc_get_modem_info() was already checked before calling
this function and also if "ch" is not NULL, dgnc_get_mstat() returns
valid value so it doesn't need to check an error.

Signed-off-by: Daeseok Youn <daeseok.youn@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>