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

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 <[email protected]>
Cc: Greg Kroah-Hartman <[email protected]>
Cc: Jens Axboe <[email protected]>
Cc: Grant Likely <[email protected]>
Cc: Rob Herring <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>