drivers: timer: nrf_rtc_timer: Fix assert conditions

This commit fixes the incomplete assert conditions for the `chan`
argument passed to the nRF RTC timer functions.

Note that the `chan` argument for this driver is of a **signed**
integer type, so it is necessary to check that its value is
non-negative.

This fixes the warnings generated by the GCC 12 such as:

  error: array subscript -1 is below array bounds of '...'

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>