xen/events/fifo: remove a unecessary use of BM()

Since 05a812ac (xen/events/fifo:
correctly align bitops), ready is an unsigned long instead of uint32_t
and the BM() macro is no longer required.

Signed-off-by: Frediano Ziglio <frediano.ziglio@citrix.com>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>