V4L/DVB (7937): pvrusb2: Change several embedded timer constants to defined values

This is primarily a cosmetic change to make it easier to change some
of the time constants used in the driver.

Signed-off-by: Mike Isely <isely@pobox.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>