mhi: core: Increase RDDM timeout to 250ms

Increase rddm_timeout_ms to 250ms as time taken for HST
to enter RDDM mode is taking more than 200ms.

Change-Id: Ibbd568261ff70997653bfc8a498df73e07e93b1d
Signed-off-by: Vivek Pernamitta <vpernami@codeaurora.org>