Input: xilinx_ps2 - various cleanups

Review comments were incorporated to improve the driver.

1. Some data was eliminated that was not needed.
2. Renaming of variables for clarity.
3. Removed unneeded type casting.
4. Changed to use dev_err rather than other I/O.
5. Merged together some functions.
6. Added kerneldoc format to functions.

Signed-off-by: Sadanand <sadanan@xilinx.com>
Signed-off-by: John Linn <john.linn@xilinx.com>
Acked-by: Peter Korsgaard <jacmet@sunsite.dk>
Acked-by: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>