ARM: PL08x: remove unnecessary NULL and BUG checks

The tasklet always is initialized with a non-NULL data argument.  It
is not possible for it to be called with a NULL data argument (unless
something is very wrong in the tasklet code - in which case lots of
stuff will break).  Therefore, as plchan can never be NULL, remove
this unnecessary BUG check.

In pl08x_tasklet(), we've already dereferenced plchan->at, so it can't
be NULL here.  Remove this unnecessary BUG check.

pl08x_fill_llis_for_desc() and pl08x_free_txd() are always called with
a non-NULL txd argument - either as a consequence of the code paths or
as a result of other checks already in place.  We don't need to repeat
the non-NULL check in these functions.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Acked-by: Linus Walleij <linus.walleij@stericsson.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>