V4L/DVB (12344): em28xx: fix support for Plextor ConvertX PX-TV100U

This device uses msp34xx and uses 2.048 MHz frequency for I2S
communication.

Thanks to Angelo Cano <acano@fastmail.fm> for pointing the issues with
this device and proposing an approach for fixing the issue.

Tested-by: Angelo Cano <acano@fastmail.fm>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>