[media] em28xx: adjust period size at runtime

While the current hardcoded period is ok for the current values,
we may latter change the driver to work with different bit rates
or with different latencies than 64ms.

So, adust the period size at runtime.

Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>

diff --cc drivers/media/usb/em28xx/em28xx-audio.c
Simple merge

diff --cc drivers/media/usb/em28xx/em28xx.h
Simple merge