ctl: add an API to set dimension levels to element information

ctl: add an API to set dimension levels to element information

In a former commit, 'struct snd_ctl_elem_info' is used as a 'container' to
transfer extra fields of element information for APIs to add an element
set. The extra fields should be filled in advance of call of the APIs.
Currently, dimension level is in the extra fields and no APIs to set it.

This commit adds an API to set dimension level to the information
structure. This API is expected to be used in advance of usage of APIs
to add an element set, for nothing others. When the information structure
is extended in future, then the similar APIs shall be added for the new
feature.

Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: Takashi Iwai <tiwai@suse.de>