staging: rtl8723au: Fix sparse warning: cast to restricted __le16

usPtr is used as __le16 *, but was defined as u16 *.
This was reported by sparse as:
drivers/staging/rtl8723au/hal/rtl8723a_hal_init.c:1850:29: warning: cast to
restricted __le16

This patch fixes the type of usPtr.

Signed-off-by: Johannes Postma <jgmpostma@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>