mISDN: remove unnecessary test on f_pos

This test is not doing anything since it is always false if the
mISDN_read() is called from vfs_read().  Besides that the driver uses
nonseekable_open() and is not using off or file->f_pos anywhere.

Signed-off-by: Jan Blunck <jblunck@suse.de>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Karsten Keil <isdn@linux-pingi.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>