can: mcp251x: Replace power callbacks with regulator API

This patch replaces power callbacks to the regulator API. To improve
the readability of the code, helper for the regulator enable/disable
was added.

Acked-by: Haojian Zhuang <haojian.zhuang@gmail.com>
Signed-off-by: Alexander Shiyan <shc_work@mail.ru>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>