Xilinx: hwicap: cleanup polling timeout.

In order to avoid polling forever if an error occurs, this driver
includes a timeout counter.  However, in fast systems, this counter
wasn't high enough.  This patch fixes the bug and also makes the
buffer-based and fifo-based drivers return the same error condition on
a timeout (-EIO).

Signed-off-by: Stephen Neuendorffer <stephen.neuendorffer@xilinx.com>