modem: modem_cmux: Set C/R bit in UIH frames

This commit sets the C/R (command/response) bit when UIH
CMUX frames are sent from the modem_cmux module. This bit
is ignored by some modems like the Quectel BG95, as there
is no defined response to this specific CMUX frame type.
However, other modems, like the TELIT ME910, require the
bit to be set (command). If the bit is not set, the modem
will simply ignore the frame completely.

Signed-off-by: Bjarki Arge Andreasen <bjarkix123@gmail.com>