modem: cmux: fix frame data length encoding

In cases where the data is bigger than 127 bytes,
the first bit of the second byte of the data length field
used to always be set.

This is wrong as according to the 3GPP 27.010 spec
only the first bit of the first byte is the EA bit;
all the others denote the data length.

Signed-off-by: Tomi Fontanilles <tomi.fontanilles@nordicsemi.no>