drivers/block/xsysace.c: fix id with missing port-number

If the port number is missing from the device-tree the device gets named
xs` instead of xsa.  This fixes the check for missing ids.

Tested on ml507 board.

Signed-off-by: Gernot Vormayr <gvormayr@gmail.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Grant Likely <grant.likely@linaro.org>
Cc: Rob Herring <rob.herring@calxeda.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>