rtlwifi: rtl8192cu: Calculate descriptor checksum correctly for BE

This driver requires a checksum for the descriptors so that the wifi
chip is assured that the USB transmission was correct. These entries
are little-endian, but the driver was always using cpu order in the
calculation. As a result, the driver failed on BE hardware.

Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
Cc: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>