Add condition for checking size_t overflow. This fixes memtest fail
> Memtest failed. Error: -22
which appears when 4GB RAM is present.
Was tested on phyFLEX-i.MX6 modules with 4GB and 1GB RAM.
Signed-off-by: Dmitry Lavnikevich <d.lavnikevich@sam-solutions.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>