libertas: rework event subscription

This patch moves re-works the implementation of event subscription
via debugfs. For this:

* it tells cmd.c and cmdresp.c about CMD_802_11_SUBSCRIBE_EVENT
* removes lots of low-level cmd stuff from debugfs.c
* create unified functions to read/write snr, rssi, bcnmiss and
  failcount
* introduces #define's for subscription event bitmask values
* add a function to search for a specific element in an IE
  (a.k.a. TLV)
* add a function to find out the size of the TLV. This is needed
  because lbs_prepare_and_send_command() has an argument for a
  data buffer, but not for it's lengths and TLVs can be, by
  definition, vary in size.
* fix a bug where it was not possible to disable an event

Signed-off-by: Holger Schurig <hs4233@mail.mn-solutions.de>
Acked-by: Dan Williams <dcbw@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>