staging: usbip: usbip_host_driver.c: avoid assignment in if

Fix coding style issue "do not use assignment in if condition"
detected by checkpatch.pl in usbip_host_driver.c.

Signed-off-by: Wahib Faizi <wahibfaizi@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>