docs: writing-an-alsa-driver.rst: fix some bad c:func: markups

Some such markups are invalid, as reported by Sphinx:

	./Documentation/sound/kernel-api/writing-an-alsa-driver.rst:3317: 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:3917: WARNING: Unparseable C cross-reference: 'copy_from/to_user'
	Invalid C declaration: Expected end of definition. [error at 9]
	  copy_from/to_user
	  ---------^

The first case seems to be better replaced by a literal.

For the second one, let's generate cross-references, by
spliting it in two.

Reviewed-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>