[ALSA] mpu401 - Add MPU401_INFO_UART_ONLY bitflag

Added MPU401_INFO_UART_ONLY bitflag to avoid issueing UART_ENTER command
at opening streams.  Some devices support only UART mode and give errors
to UART_ENTER.
A new module option, uart_enter, is added to snd-mpu401 driver.
For UART-only devices, set uart_enter=0.

Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@suse.cz>