media: mediatek: vcodec: Define address for VDEC_HW_ACTIVE

The VDEC_HW_ACTIVE bit is located at offset 0, bit 4 of the VDECSYS
iospace. Only the mask was previously defined, with the address being
implicit. Explicitly define the address, and append a '_MASK' suffix to
the mask, to make accesses to this bit clearer.

This commit brings no functional change.

Signed-off-by: Nícolas F. R. A. Prado <nfraprado@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>