ec_host_cmd: Fix generating multiple Port80 notifications

Port80 notifications are continously generated as long as NOT_EMPTY
bit inside of Data Attributes register is set. This register was only
read once prior entering loop and its value was not checked on each
iteration.

This patch will include reading Data Attributes register on each
iteration, this way we can exit loop when no more data is available.

Signed-off-by: Bernardo Perez Priego <bernardo.perez.priego@intel.com>