char: xilinx_hwicap: Fix typo in comment and extend it

s/regsiter/register/

Use origin comment from the first driver version
which also explain the usage of XHI_MAX_RETRIES better.

Signed-off-by: Michal Simek <monstr@monstr.eu>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>