tty: n_gsm: add helpers to convert mux-num to/from tty-base

Make it obvious how the gsm mux number relates to the virtual tty lines
by using helper functions instead of shifting 6 bits.

Signed-off-by: Martin Hundebøll <martin@geanix.com>
Link: https://lore.kernel.org/r/20190710192656.60381-3-martin@geanix.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>