Invalid value set in uart_init()

[rwalle]

According to BCM2711 datasheet

https://github.com/babbleberry/rpi4-osdev/blob/c9a2a1429ae7cf0f6d1632c3f4a92fb8457e6c07/part3-helloworld/io.c#L74

Documentation for AUX_MU_LCR_REG says:

Therefore 3 (0b11) is not a valid value, and the correct value should be 1.

https://github.com/babbleberry/rpi4-osdev/blob/c9a2a1429ae7cf0f6d1632c3f4a92fb8457e6c07/part3-helloworld/io.c#L77

0xC6 is not a valid value and does not disable interrupts -- it clears both FIFOs, and the correct value to use is 0x06.