V4L/DVB (13586): DiB0700: Add parameter to change the buffer size

Add parameter to change the buffer size. This buffer size is specified
in number of Ts packet. This parameter is stored inside the state.

For firmware higher than 1.21, the xfer buffer size can be changed
inside the dib0700 usb bridge the firware version is stored inside the
state

Signed-off-by: Patrick Boettcher <pboettcher@kernellabs.com>
Signed-off-by: Olivier Grenie <olivier.grenie@dibcom.fr>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>