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>

Documentation/sound/kernel-api/writing-an-alsa-driver.rst

@@ -3315,8 +3315,7 @@ data and removes them from the buffer at once:
   }
 
 If you know beforehand how many bytes you can accept, you can use a
-buffer size greater than one with the
-:c:func:`snd_rawmidi_transmit\*()` functions.
+buffer size greater than one with the ``snd_rawmidi_transmit*()`` functions.
 
 The ``trigger`` callback must not sleep. If the hardware FIFO is full
 before the substream buffer has been emptied, you have to continue
@@ -3916,7 +3915,7 @@ the maximum size of the proc file access.
 
 The read/write callbacks of raw mode are more direct than the text mode.
 You need to use a low-level I/O functions such as
-:c:func:`copy_from/to_user()` to transfer the data.
+:c:func:`copy_from_user()` and :c:func:`copy_to_user()` to transfer the data.
 
 ::