atmel-mci: Fix bogus debugfs file size

We used to store a binary register snapshot in the "regs" file, so we
set the file size to be the size of this snapshot. This is no longer
valid since we switched to using seq_file.

Signed-off-by: Haavard Skinnemoen <haavard.skinnemoen@atmel.com>
Signed-off-by: Pierre Ossman <drzeus@drzeus.cx>