drivers/modem/modem_iface_uart: Update API

The UART IFACE API currently exposes the context struct
modem_iface_uart_data, expecting the user to fill in some
of the fields, with no documentation specifying which fields
and what they mean.

This API update moves all user configurable values in the context
out into a config struct, modem_iface_uart_config, within which
members are documented.

This prevents the user from interacting directly with the context
making use of the library safer and more readable.

The config structure helps make code readable by using "named args"
in a const struct instead of a long, nameless, parameter list passed
to the modem_iface_uart_init function.

The new API function modem_iface_uart_rx_wait is added to prevent the
user from having to interact with the rx sem in the context directly.

The context can now be safely interated with without direct access to
any of its members.

Pointers to the ring buffer params in the context have been moved to
config struct, as these are only useful during initialization of the
context.

Signed-off-by: Bjarki Arge Andreasen <baa@trackunit.com>