drivers: modem: gsm: Go to next state if mux_attach successfully

Currently the mux_setup set the state to the next one after
uart_mux_alloc is successful even if the mux_setup fails which
can be a problem.

Set the state after both mux_setup & uart_mux_alloc are
successful.

Signed-off-by: Yong Cong Sin <yongcong.sin@gmail.com>