hal qspi test: fix frequency setting test
hal qspi test: fix frequency setting test

Prevent setting frequency below QSPI_MIN_FREQUENCY

kjbracey-arm/mbed,kjbracey-arm/mbed,mbedmicro/mbed,kjbracey-arm/mbed,mbedmicro/mbed,andcor02/mbed-os,andcor02/mbed-os,mbedmicro/mbed,andcor02/mbed-os,kjbracey-arm/mbed,andcor02/mbed-os,andcor02/mbed-os,mbedmicro/mbed,andcor02/mbed-os,mbedmicro/mbed