nimble/ll: fix and refactor set rpa timeout
nimble/ll: fix and refactor set rpa timeout

os_callout_reset return value was not assigned to rc.
Additionally code was refactored to improve readability.

X-Original-Commit: f1a94d2a32b7a93bae5ef6b90e249cb4494202a0

apache/mynewt-nimble,apache/mynewt-nimble,apache/mynewt-nimble,apache/mynewt-nimble,apache/mynewt-nimble