iwlwifi: mvm: use ssize_t for len in iwl_debugfs_mem_read()

In iwl_dbgfs_mem_read(), the len variable may become negative and is
compared to < 0 (an error case).  Comparing size_t (which is unsigned)
to < 0 causes a warning on certain platforms (like i386):

drivers/net/wireless/intel/iwlwifi/mvm/debugfs.c:1561:5-8: WARNING: Unsigned expression compared with zero: len < 0

To prevent that, use ssize_t for len instead.

Fixes: commit 2b55f43f ("iwlwifi: mvm: Add mem debugfs entry")
Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>