iwlwifi: allow to limit the size of the external buffer for firmware debugging

When we use an external buffer, it is allocated from the
t DRAM and can be as big as 64MB. This buffer is huge and
might not be needed for the specific issue being chased.
Especially if lots of dumps are going to be created.
Allow to limit the size of the buffer in the configuration.

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