can: peak_usb: remove write only variable struct peak_usb_adapter::ts_period

The variable struct peak_usb_adapter::ts_period is only ever written
to. This patch removes it from the driver.

Link: https://lore.kernel.org/r/20210406111622.1874957-5-mkl@pengutronix.de
Acked-by: Stephane Grosjean <s.grosjean@peak-system.com>
Tested-by: Stephane Grosjean <s.grosjean@peak-system.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>