drivers: spi: cc13xx_cc26xx: Allow clocks below 2 MHz

Remove unnecessary check that the SPI clock is being set to a frequency
above 2 MHz to allow devices running at common lower frequencies (i.e.
1.2 MHz and 400 kHz).

Replace with check that the frequency is not below the minimum frequency
supported by the chipset to prevent overflow error which can occur if
the HAL sets a frequency too low resulting in a SPI clock much larger
than expected.

Fixes #69986

Signed-off-by: David Lim <dlim04@qub.ac.uk>