USB: cxacru: Fix negative dB output

Values of dB between -0.99 and -0.01 will be output with the wrong
sign. This converts the negative value to positive and outputs it
with a "-" prefix.

Signed-off-by: Simon Arlott <simon@fire.lp0.eu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>