docs: sound: writing-an-alsa-driver.rst: get rid of :c:type

the :c:type shouldn't be used with structs with Sphinx 3,
as the C domain there uses .. c:struct for structs.

As we have the automarkup extension, let's just get rid of
all :c:type as a whole, as those will be automagically
marked as such.

This solves a bunch of warnings with Sphinx 3, like those:

	.../Documentation/sound/kernel-api/writing-an-alsa-driver.rst:490: WARNING: Unparseable C cross-reference: 'calling snd_card_free'
	Invalid C declaration: Expected end of definition. [error at 8]
	  calling snd_card_free
	  --------^
	.../Documentation/sound/kernel-api/writing-an-alsa-driver.rst:3328: WARNING: Unparseable C cross-reference: 'snd_rawmidi_transmit*'
	Invalid C declaration: Expected end of definition. [error at 20]
	  snd_rawmidi_transmit*
	  --------------------^
	.../Documentation/sound/kernel-api/writing-an-alsa-driver.rst:3928: WARNING: Unparseable C cross-reference: 'copy_from/to_user'
	Invalid C declaration: Expected end of definition. [error at 9]
	  copy_from/to_user
	  ---------^
Reviewed-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>