USB backend: Fix sign issues reported by Visual Studio
USB backend: Fix sign issues reported by Visual Studio

Considering this:

unsigned int foo;
int bar() { return -foo; }

Visual Studio considers that the arithmetic negation unary operator
applied on an unsigned value will returns an unsigned value, and will
issue an warning about it.

Signed-off-by: Paul Cercueil <73d292422978d45c9eeef455bb1b24a0db453722@analog.com>

analogdevicesinc/libiio,gburca/libiio,gburca/libiio,gburca/libiio,sensarliar/libiio,analogdevicesinc/libiio,analogdevicesinc/libiio,sensarliar/libiio,analogdevicesinc/libiio,sensarliar/libiio