input: sensors: change akm09911 measure mode to continuous

Continuous mode doesn't need to send SNG_MEASUREMENT command. It
can save some cpu cycles and reach higher polling rate.

CRs-fixed: 719393
Change-Id: I5abaae4050e58dc1ddf9265adfe84a7a34aed3d9
Signed-off-by: Oliver Wang <mengmeng@codeaurora.org>