plugins/fmcomms2.c: UP/DN Converter Fix frequency upper limits

Signed-off-by: Dan Nechita <dan.nechita@analog.com>
plugins/fmcomms2.c: Don't overwrite UP/DN freq with PLL freq when reloading
RX and TX LO frequencies of the UP/DN converter were getting overwritten with RX and TX values of the PLL when after clicking the "Reload Settings" button.

Signed-off-by: Dan Nechita <dan.nechita@analog.com>