i2c: fix stack buffer overflow vulnerability in i2c md command

When running "i2c md 0 0 80000100", the function do_i2c_md parses the
length into an unsigned int variable named length. The value is then
moved to a signed variable:

    int nbytes = length;
    #define DISP_LINE_LEN 16
    int linebytes = (nbytes > DISP_LINE_LEN) ? DISP_LINE_LEN : nbytes;
    ret = dm_i2c_read(dev, addr, linebuf, linebytes);

On systems where integers are 32 bits wide, 0x80000100 is a negative
value to "nbytes > DISP_LINE_LEN" is false and linebytes gets assigned
0x80000100 instead of 16.

The consequence is that the function which reads from the i2c device
(dm_i2c_read or i2c_read) is called with a 16-byte stack buffer to fill
but with a size parameter which is too large. In some cases, this could
trigger a crash. But with some i2c drivers, such as drivers/i2c/nx_i2c.c
(used with "nexell,s5pxx18-i2c" bus), the size is actually truncated to
a 16-bit integer. This is because function i2c_transfer expects an
unsigned short length. In such a case, an attacker who can control the
response of an i2c device can overwrite the return address of a function
and execute arbitrary code through Return-Oriented Programming.

Fix this issue by using unsigned integers types in do_i2c_md. While at
it, make also alen unsigned, as signed sizes can cause vulnerabilities
when people forgot to check that they can be negative.

Signed-off-by: Nicolas Iooss <nicolas.iooss+uboot@ledger.fr>
Reviewed-by: Heiko Schocher <hs@denx.de>