drivers: serial: nrfx_uarte: Fix spurious RXTO event

Fast instance in nrf54h (uart120) can generate a spurious RXTO event
some time after RXTO event that indicates that RX path is disabled.
The time when event is generated depends on baudrate and when slower
baudrates are used peripheral is disabled on time to not notice it
in the test but with higher baudates issue become visible. In order
to avoid spurious interrupt, RXTO interrupt is disabled during RXTO
event handling and enabled when RX is enabled. This workaround is
applied only for fast instance to avoid unnecessary register
accesses for slower instances.

Signed-off-by: Krzysztof Chruściński <krzysztof.chruscinski@nordicsemi.no>