ALSA: seq: More constifications

Apply const prefix to the remaining sequencer code: the static tables
for MIDI macros, RPN/NRPN, and some strings.

Just for minor optimization and no functional changes.

Link: https://lore.kernel.org/r/20200105144823.29547-7-tiwai@suse.de


Signed-off-by: Takashi Iwai <tiwai@suse.de>