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

drivers/firmware/cirrus/cs_dsp.c

@@ -758,6 +758,9 @@ int cs_dsp_coeff_write_ctrl(struct cs_dsp_coeff_ctl *ctl, const void *buf, size_
 
 	lockdep_assert_held(&ctl->dsp->pwr_lock);
 
+	if (!ctl)
+		return -ENOENT;
+
 	if (ctl->flags & WMFW_CTL_FLAG_VOLATILE)
 		ret = -EPERM;
 	else if (buf != ctl->cache)
@@ -817,6 +820,9 @@ int cs_dsp_coeff_read_ctrl(struct cs_dsp_coeff_ctl *ctl, void *buf, size_t len)
 
 	lockdep_assert_held(&ctl->dsp->pwr_lock);
 
+	if (!ctl)
+		return -ENOENT;
+
 	if (ctl->flags & WMFW_CTL_FLAG_VOLATILE) {
 		if (ctl->enabled && ctl->dsp->running)
 			return cs_dsp_coeff_read_ctrl_raw(ctl, buf, len);