firmware: Avoid caching firmware when FW_OPT_NOCACHE is set

When calling request_firmware_into_buf(), we pass the FW_OPT_NOCACHE
flag with the intent of skipping the caching mechanism of the
firmware loader. Unfortunately, that doesn't work, because
fw_lookup_and_allocate_buf() isn't told to _not_ add the struct
firmware_buf to the firmware cache (fwc) list. So when we call
request_firmware_into_buf() the second time, we find the buffer in
the cache and return it immediately without reloading.

This may break users of request_firmware_into_buf that are expecting
a fresh copy of the firmware to be reloaded into memory. The existing
copy may either be modified by drivers, remote processors or even
freed. Fix fw_lookup_and_allocate_buf to not add to the fwc list
if FW_OPT_NOCACHE is set, and also don't do the lookup in the list.

Fixes: 0e742e9271 ("firmware: provide infrastructure to make fw caching optional")

Change-Id: I39fa4657ab5993b2b8e39ac65000dfc1640db866
Signed-off-by: Vikram Mulukutla <markivx@codeaurora.org>
Signed-off-by: Rishabh Bhatnagar <rishabhb@codeaurora.org>