[ALSA] cmipci: check that the legacy MIDI port works

Check that the UART_EN bit actually enabled the MPU-401 port.
Apparently, C-Media thinks that it is a good idea to be paranoid here.

Signed-off-by: Clemens Ladisch <clemens@ladisch.de>
Signed-off-by: Jaroslav Kysela <perex@suse.cz>