xen/events/fifo: correctly align bitops

FIFO event channels require bitops on 32-bit aligned values (the event
words).  Linux's bitops require unsigned long alignment which may be
64-bits.

On arm64 an incorrectly unaligned access will fault.

Fix this by aligning the bitops along with an adjustment for bit
position and using an unsigned long for the local copy of the ready
word.

Cc: stable@vger.kernel.org
Signed-off-by: Vladimir Murzin <murzin.v@gmail.com>
Tested-by: Pranavkumar Sawargaonkar <pranavkumar@linaro.org>
Reviewed-by: Ian Campbell <ian.campbell@citrix.com>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>