iwlwifi: pcie: fix prph dump length

The length counting previously done had an error in it, causing
the length down the data dumping function to be shorter than it
should be, causing the end of the data to get truncated off and
lost.

Cc: <stable@vger.kernel.org> [3.17+]
Fixes: 67c65f2c ("iwlwifi: dump periphery registers to fw-error-dump")
Signed-off-by: Liad Kaufman <liad.kaufman@intel.com>
Reviewed-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>