rtlwifi: rtl8192cu: remove pointless memcpy

gcc-8 points out that source and destination of the memcpy() are
always the same pointer, so the effect of memcpy() is undefined
here (its arguments must not overlap):

drivers/net/wireless/realtek/rtlwifi/rtl8192cu/trx.c: In function '_rtl_rx_process':
drivers/net/wireless/realtek/rtlwifi/rtl8192cu/trx.c:430:2: error: 'memcpy' source argument is the same as destination [-Werror=restrict]

Most likely this is harmless, but it's easy to just remove the
line and get rid of the warning.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>