s/regsiter/register/

Use origin comment from the first driver version
which also explain the usage of XHI_MAX_RETRIES better.
Signed-off-by: NMichal Simek <monstr@monstr.eu>
Signed-off-by: NMichal Simek <michal.simek@xilinx.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>