firmware: cs_dsp: Perform NULL check in cs_dsp_coeff_write/read_ctrl

Add a NULL check to the cs_dsp_coeff_write/read_ctrl functions. This is
a major convenience for users of the cs_dsp library as it allows the call
to cs_dsp_get_ctl to be inlined with the call to read/write the control
itself.

Signed-off-by: Charles Keepax <ckeepax@opensource.cirrus.com>
Link: https://lore.kernel.org/r/20211117132300.1290-7-ckeepax@opensource.cirrus.com


Signed-off-by: Mark Brown <broonie@kernel.org>