Revert "firmware_loader: use kernel credentials when reading firmware"

This reverts commit 3677563e

 as it leaks
memory :(

Reported-by: Qian Cai <quic_qiancai@quicinc.com>
Link: https://lore.kernel.org/r/20220427135823.GD71@qian
Cc: Thiébaud Weksteen <tweek@google.com>
Cc: Luis Chamberlain <mcgrof@kernel.org>
Cc: John Stultz <jstultz@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>