caif: remove unused struct member

The tty_name member of struct ser_device is never set or used, so it
can be removed. (The definition of struct ser_device is private to
this .c file, and the identifier tty_name only occurs in this one
place.)

Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Signed-off-by: David S. Miller <davem@davemloft.net>