drivers: gnss: nmea_generic: some fixes/improvements

- Configure a UART TX buffer; the backend would otherwise assert in
its configuration or (if the asserts are off) miserably fail when
trying to send anything to the GNSS modem.
- Fine tune the UART RX buffer size and make it depend on whether
satellite data is received.
- Remove unused k_spinlock.
- Make declaration of Kconfig items dependent on GNSS_NMEA_GENERIC
conditional.

Signed-off-by: Tomi Fontanilles <tomi.fontanilles@nordicsemi.no>