modem: cmux: replace `crc8` with `crc8_rohc` for FCS calculation

To improve performance in FCS calculation, this commit replaces
the usage of the generic `crc8` function with the specific `crc8_rohc`
function in `modem_cmux.c`.
The `crc8_rohc` function utilizes a small table approach, enhancing
the efficiency of CRC-8/ROHC variant calculations while optimizing
memory usage.

Signed-off-by: Pisit Sawangvonganan <pisit@ndrsolution.com>