fifo: Remove shared variable

fifo: Remove shared variable

The shared variable used for FIFO memory is not VHDL 2008 compliant.
I can't see why it needs to be a shared variable since reads and writes
update top and bottom synchronously, meaning they don't need same cycle
access to the FIFO memory.

Signed-off-by: Anton Blanchard <anton@linux.ibm.com>