iwlwifi: mvm: make debugfs write() operations write up to count bytes

Some debugfs write() operations of the MVM Firmware will ignore the
count argument, and will copy more bytes than what was specified.
Fix this by getting the right count of bytes.

This will honor restrictions put on the number of bytes to write and
avoid strcmp() calls on garbage data.

Signed-off-by: Djalal Harouni <tixxdz@opendz.org>
Reviewed-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>