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

drivers/char/xilinx_hwicap/xilinx_hwicap.h

@@ -85,7 +85,13 @@ struct hwicap_driver_config {
 	void (*reset)(struct hwicap_drvdata *drvdata);
 };
 
-/* Number of times to poll the done regsiter */
+/* Number of times to poll the done register. This has to be large
+ * enough to allow an entire configuration to complete. If an entire
+ * page (4kb) is configured at once, that could take up to 4k cycles
+ * with a byte-wide icap interface. In most cases, this driver is
+ * used with a much smaller fifo, but this should be sufficient in the
+ * worst case.
+ */
 #define XHI_MAX_RETRIES     5000
 
 /************ Constant Definitions *************/