Fix constant conversion warnings in uplcom(4)

[dim]

Summary

With clang 3.9.0, compiling uplcom results in the following warnings:

sys/dev/usb/serial/uplcom.c:543:29: error: implicit conversion from 'int' to 'int8_t' (aka 'signed char') changes value from 192 to -64 [-Werror,-Wconstant-conversion]
        if (uplcom_pl2303_do(udev, UT_READ_VENDOR_DEVICE, UPLCOM_SET_REQUEST, 0x8484, 0, 1)
            ~~~~~~~~~~~~~~~~       ^~~~~~~~~~~~~~~~~~~~~
sys/dev/usb/usb.h:179:53: note: expanded from macro 'UT_READ_VENDOR_DEVICE'
#define UT_READ_VENDOR_DEVICE   (UT_READ  | UT_VENDOR | UT_DEVICE)
                                 ~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~

This is because UT_READ is 0x80, so the int8_t argument is wrapped to a
negative value. While this should not effect the eventual bits in the
destination (which still end up correctly), it is better to use uint8_t
instead.