PM / devfreq: bw_hwmon: Fix parsing of mbps_zones

store_list_mbps_zones() uses kstrtoint() to parse the list of
integers. kstrtoint() accepts only null or newline terminated
strings as inputs and thus trying to echo a space separated list of
values into mbps_zones fails.
This patch fixes the parsing logic.

Change-Id: Ibc3e34638b589846a4ee6684db8d1ef09359c0ab
Signed-off-by: Rohit Gupta <rohgup@codeaurora.org>