drivers/staging/bcm: Integer overflow

The checking condition in 'validateFlash2xReadWrite()' is not
sufficient. A large number invalid would cause an integer overflow and
pass the condition, which could cause further integer overflows in
'Bcmchar.c:bcm_char_ioctl()'.

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