atm: idt77252, use setup_timer and mod_timer

Stop accessing timer struct members directly and use setup_timer and
mod_timer helpers intended for that use. It makes the code cleaner and
will allow for easier change of the timer struct internals.

Signed-off-by: Jan Koniarik <jan.koniarik@trustica.cz>
Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Cc: Chas Williams <3chas3@gmail.com>
Cc: <linux-atm-general@lists.sourceforge.net>
Cc: <netdev@vger.kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>