OPP: fix dev_pm_opp_find_bw_*() when bandwidth table not initialized

If a driver calls dev_pm_opp_find_bw_ceil/floor() the retrieve bandwidth
from the OPP table but the bandwidth table was not created because the
interconnect properties were missing in the OPP consumer node, the
kernel will crash with:

Unable to handle kernel NULL pointer dereference at virtual address 0000000000000004
...
pc : _read_bw+0x8/0x10
lr : _opp_table_find_key+0x9c/0x174
...
Call trace:
  _read_bw+0x8/0x10 (P)
  _opp_table_find_key+0x9c/0x174 (L)
  _find_key+0x98/0x168
  dev_pm_opp_find_bw_ceil+0x50/0x88
...

In order to fix the crash, create an assert function to check
if the bandwidth table was created before trying to get a
bandwidth with _read_bw().

Fixes: add1dc09 ("OPP: Use generic key finding helpers for bandwidth key")
Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>