staging: rtl8723au: include/rtl8723a_hal.h: use __leXX types in fw header struct

This patch makes fields in struct rt_8723a_firmware_hdr use
endianness-aware types (__leXX), thus fixing the following sparse warnings:

CHECK   drivers/staging/rtl8723au/hal/rtl8723a_hal_init.c
drivers/staging/rtl8723au/hal/rtl8723a_hal_init.c:263:37: warning: cast to
restricted __le16
drivers/staging/rtl8723au/hal/rtl8723a_hal_init.c:265:39: warning: cast to
restricted __le16
drivers/staging/rtl8723au/hal/rtl8723a_hal_init.c:277:13: warning: cast to
restricted __le16
drivers/staging/rtl8723au/hal/rtl8723a_hal_init.c:277:13: warning: cast to
restricted __le16
drivers/staging/rtl8723au/hal/rtl8723a_hal_init.c:277:13: warning: cast to
restricted __le16

Signed-off-by: Daniele Alessandrelli <daniele.alessandrelli@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>