HID: cp2112: fix incorrect error propagation in cp2112_xfer()

Both cp2112_read_req() and cp2112_write_req() are returning negative
value in cases of error, but cp2112_xfer() is storing the return
value into unsigned size_t-typed 'count'.

Fix this by making 'count' signed type.

Reported-by: fengguang.wu@intel.com
Signed-off-by: Jiri Kosina <jkosina@suse.cz>