video: mb862xx-i2c: local functions should not be exposed globally

Functions not referenced outside of a source file should be marked
static to prevent them from being exposed globally.

Quiets the sparse warnings:

warning: symbol 'mb862xx_i2c_stop' was not declared. Should it be static?

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Cc: Anatolij Gustschin <agust@denx.de>
Signed-off-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>