dbsrx2: Fixed rounding bug when tuning close to integer ratio transition...

If the target LO frequency is close to an integer multiple of the reference frequency, the fractional divide ratio will round to (1 << 20).  This value is too large MAX2112 fractional divide register, resulting in an error in the LO frequency of approximately ref_freq.  GNURadio's float_converter() will often translate 976e6 to 975999999.99999988079071044921875, causing frequent issues when using a USRP1 w/ DBSRX2.  

Added a check for this edge case.