greybus: uart: Update uart.c to register tty ports

For each new UART connection we need to do a tty_port_init else
we'll crash when trying to access the tty mutex later on.
Base the TTY major/minor numbers on non-zero values.
Supply an empty operations structure for the newly regitered port.

Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>