firmware: New fractional sample rate algorithm and usb command

Signed-off-by: Sylvain Munaut <tnt@246tNt.com>
libhackrf: Update the library with new fractional sample rate API

And send the new USB command

Signed-off-by: Sylvain Munaut <tnt@246tNt.com>
lib/fw: rename the sample rate function
lib: make sure to tell the linker that we're hardfloat, or it will choose the wrong libgcc.a...
tools: operator precedence, precision
Merge pull request #76 from smunaut/pending

Final (hopefully) sample rate API + misc fixes