diag: Add HSIC communication support

Add support for relaying the MDM 9k diag traffic over HSIC
to the USB DIAG MDM channel and vice versa. Also modify usb
diag_bridge to use signed parameters for buffer sizes. This
is especially needed for the final parameter of the read
and write callbacks, wherein a negative errno value may be
passed to convey an error.

Change-Id: Ifa516356a5d72597d82929a56d714f8dfb2964de
Signed-off-by: Dixon Peterson <dixonp@codeaurora.org>