tx493xide: use min_t() macro instead of min()

This fixes a warning ("comparison of distinct pointer types lacks a
cast") introduced by the commit
040f6b4f ("tx493xide: use ->pio_mode
value to determine pair device speed").

Signed-off-by: Atsushi Nemoto <anemo@mba.ocn.ne.jp>
Signed-off-by: David S. Miller <davem@davemloft.net>