drm/i2c: tda998x: simplify the i2c read/write functions

This patch simplifies the i2c read/write functions and permits them to
be easily called in more contexts.

Tested-by: Russell King <rmk+kernel@arm.linux.org.uk>
Acked-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Jean-Francois Moine <moinejf@free.fr>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>