mxcmmc: fixed max_seg_size value on initialization

This unpleasant typo appeared while porting the driver from Freescale
original sources, where anyone can easily find the correct version.

Current incorrect version potentially can influence segment and merge
handling in block subsystem via MMC request queue settings.

Signed-off-by: Vladimir Zapolskiy <vzapolskiy@gmail.com>
Cc: Sascha Hauer <s.hauer@pengutronix.de>
Cc: Pierre Ossman <pierre@ossman.eu>
Cc: <linux-mmc@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>