Usb: Fix open/close pipe functions never returning errors on MSVC
Usb: Fix open/close pipe functions never returning errors on MSVC

Under MSVC, the minus unary operator applied to an unsigned value
results in an unsigned value. Then, we need to cast the unsigned
value to a signed value before we apply the minus operator.

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

analogdevicesinc/libiio,analogdevicesinc/libiio,analogdevicesinc/libiio,analogdevicesinc/libiio