EXT4: Fix number base handling of "ext4write" command

EXT4: Fix number base handling of "ext4write" command

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 b770e88

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>