drivers: video: common: avoid dead code in functions with retry

Avoid k_sleep() never being called when video_write_reg_retry() and
video_read_reg_retry() have a number of retry set to zero (default).
The default number of retries being controlled by
CONFIG_VIDEO_I2C_RETRY_NUM.

Signed-off-by: Josuah Demangeon <me@josuah.net>