z055_hdlc_drv: Update function callback API to compile with kernel >= 6.8

In kernel 6.8, they have changed the definition of a function pointer within the struct tty_operations. The member send_xchar has changed from 'void (*)(struct tty_struct , char)' to 'void ()(struct tty_struct *, u8)'

Here the commit where the change was added:

3a00da02 (tty: make tty_operations::send_xchar accept u8 char)