media: ddbridge: split I/O related functions off from ddbridge.h

While it seems valid that headers can carry simple oneline static inline
annotated functions, move them into their own header file to have the
overall code more readable. Also, keep them as header (and don't put in
a separate object) and static inline to help the compiler avoid
generating function calls.

(Thanks to Jasmin J. <jasmin@anw.at> for valuable input on this!)

Cc: Jasmin J. <jasmin@anw.at>
Signed-off-by: Daniel Scheller <d.scheller@gmx.net>
Tested-by: Richard Scobie <r.scobie@clear.net.nz>
Tested-by: Jasmin Jessich <jasmin@anw.at>
Tested-by: Dietmar Spingler <d_spingler@freenet.de>
Tested-by: Manfred Knick <Manfred.Knick@t-online.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>