[ALSA] mpu-401: remove MPU401_INFO_UART_ONLY flag

Since the last patch made the ENTER_UART command optional, the
enter_uart option and its corresponding flag have become superfluous.
The uart_enter option remains for backward compatibility but just prints
a warning when used.

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