fbdev/xilinxfb: Fix improper casting and tighen up probe path

The xilinxfb driver is improperly casting a physical address to a
u32, and the probe routine isn't as straight forward as it could be.
(discovered by gcc spitting out warnings on most recent change to
xilinxfb driver).

This patch fixes the cast and simplifies the probe path.

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
Tested-by: John Linn <john.linn@xilinx.com>