media: drxk_hard: check if parameter is not NULL

There is a smatch warning:
	drivers/media/dvb-frontends/drxk_hard.c: drivers/media/dvb-frontends/drxk_hard.c:1478 scu_command() error: we previously assumed 'parameter' could be null (see line 1467)

Telling that parameter might be NULL. Well, it can't, due to the
way the driver works, but it doesn't hurt to add a check, in order
to shut up smatch.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>