media: si2157: fix bandwidth stored in dev

Make digital tuning store the bandwidth in Hz in the private dev struct,
rather than the hardware-specific bandwidth property code, so that the
get_bandwidth() function returns the bandwidth in Hz, just as it already
does when using analog tuning.

Link: https://lore.kernel.org/linux-media/trinity-931c0e68-88af-46cc-91a1-986754798a4f-1641509499366@3c-app-gmx-bap68


Reported-by: Robert Schlabbach <robert_s@gmx.net>
Signed-off-by: Robert Schlabbach <robert_s@gmx.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>