Stop using tty internal structure in mxser_receive_chars(), use
tty_insert_flip_char(tty, ch flag); instead.

Without this change driver ignores any rx'ed chars.

Run tested.

Cc: Alan Cox <alan@redhat.com>
Cc: Russell King <rmk@arm.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>