staging/usbip: Mark local functions as static (fix sparse warnings)

sparse complains about these functions:
usbip/stub_dev.c:529:5: warning: symbol 'stub_pre_reset' was not declared. Should it be static?
usbip/stub_dev.c:535:5: warning: symbol 'stub_post_reset' was not declared. Should it be static?

-> add static keyword to silence the warning and make sparse happy.

Signed-off-by: Peter Huewe <peterhuewe@gmx.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>