Staging: vt6655-6: shift wrap buf in s_vFillTxKey()

byKeyIndex is an unsigned char between 0 and 0xf.  If it is any value
higher than 1, then we will hit an integer wrap issue here.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>