iwlwifi: mvm: fix fw_rx_stats debugfs entry

The fw_rx_stats entry in debugfs was getting truncated because the
internal buffer used to hold the string was too short.  The
calculation of the needed buffer size was rather bogus.

Simplify the calculation by multiplying the number of entries in the
entire structure by the size of each data line and adding the size of
the header lines.

Additionally, add the mac_id value, which was missing.

Signed-off-by: Luciano Coelho <luciano.coelho@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>