staging: rtl8712: Fix possible buffer overrun

In commit 8b7a13c3 ("staging: r8712u: Fix possible buffer
overrun") we fix a potential off by one by making the limit smaller.
The better fix is to make the buffer larger.  This makes it match up
with the similar code in other drivers.

Fixes: 8b7a13c3 ("staging: r8712u: Fix possible buffer overrun")
Signed-off-by: Young Xiao <YangX92@hotmail.com>
Cc: stable <stable@vger.kernel.org>
Reviewed-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>