iwlwifi: mvm: dump to debugfs the SRAM as binary

This allows to format it at will using external tools.
Since different teams want it in different formats, dump
the raw data and everyone can play with the data the way
they want.

While at it - make this code slightly more robust by making
the required verification on the offsets / length in the
write handler.

Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>