tty: Rework receive flow control char logic

tty: Rework receive flow control char logic

Add a helper to check if the character is a flow control one. This
rework prepares for adding lookahead done check cleanly to
n_tty_receive_char_flow_ctrl() between n_tty_is_char_flow_ctrl() and
the actions taken on the flow control characters.

No functional changes intended.

Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Link: https://lore.kernel.org/r/20220426144935.54893-2-ilpo.jarvinen@linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>