refactor(drivers/marvell/comphy-3700): rename Clock Source Low value constants

The constants BUNDLE_PERIOD_SCALE and PLL_READY_DLY refer to two
multi-bit registers within the Clock Source Low register. These
constants are used as masks for those registers (and values are not
defined since we are writing zeros to them).

Give them the _MASK suffix.

Signed-off-by: Marek Behún <marek.behun@nic.cz>
Change-Id: Id469d0ab4c755d2d6a0150a1ade33dd9d0293667