Unlike other commands (for example, "fatwrite"), ext4write would
interpret the "sizebytes" as decimal number.  This is not only
inconsistend and unexpected to most users, it also breaks usage
like this:

	tftp ${addr} ${name}
	ext4write mmc 0:2 ${addr} ${filename} ${filesize}

Change this to use the standard notation of base 16 input format.
See also commit b770e88a

WARNING: this is a change to the user interface!!

Signed-off-by: Wolfgang Denk <wd@denx.de>
Cc: Uma Shankar <uma.shankar@samsung.com>
Cc: Stephen Warren <swarren@nvidia.com>