USB: usbtmc: fix timeout increase

The current 10ms timeout is too short for some normal USBTMC device
operation, increase it to a value which was tested with previously
affected Tektronix oscilloscopes.

Signed-off-by: Gergely Imreh <imrehg@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>