sensors: unify the poll delay interface name of bmp18x sensor driver

Change the bmp18x sensor poll delay interface name from "delay" to
"poll_delay", because the "poll_delay" is used as uniform interface.

Change-Id: I380c5d32518eacb9f14985424c22c43d7d985658
Signed-off-by: Jie Cheng <rockiec@codeaurora.org>