drivers: gnss: fix setting of baudrate

ubx_m10_init_pipe is calling modem_backend_uart_init which in turn is
memsetting the backend data structure and thus also overwrites the work
queue function pointer in it.

Remove call to ubx_m10_init_pipe as data structures are already set up.

Tested with u-blox NEO M9N.

Signed-off-by: Andreas Klinger <ak@it-klinger.de>