net: wireless: bcmdhd: fix buffer overrun in wl_android_set_roampref

added boundary check not to override allocated buffer.
Specially when user input corrupted or manipulated.

Signed-off-by: Insun Song <insun.song@broadcom.com>
Change-Id: Id6196da10111517696eda5f186b1e2dd19f66085
Bug: 34469904