brcmsmac: Improve tx trace and debug support

Add the brcmsmac_tx trace system for tx debugging. Existing code to dump
tx status and descriptors are converted to using tracepoints, allowing
for more efficient collection and post-processing of this data. These
tracepoints are placed to collect data for all tx frames instead of only
on errors. Logging of tx errors is also improved.

Acked-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: Seth Forshee <seth.forshee@canonical.com>
Tested-by: Daniel Wagner <wagi@monom.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>