staging: r8188eu: remove a bitwise AND from rtw_writeN()

staging: r8188eu: remove a bitwise AND from rtw_writeN()

Remove an unnecessary bitwise AND because "length" can never be greater
than 0xffff since VENDOR_CMD_MAX_DATA_LEN is defined as '254'.

Co-developed-by: Pavel Skripkin <paskripkin@gmail.com>
Signed-off-by: Pavel Skripkin <paskripkin@gmail.com>
Signed-off-by: Fabio M. De Francesco <fmdefrancesco@gmail.com>
Link: https://lore.kernel.org/r/20210924122705.3781-11-fmdefrancesco@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>