iwlwifi: pnvm: Fix a memory leak in 'iwl_pnvm_get_from_fs()'

A firmware is requested but never released in this function. This leads to
a memory leak in the normal execution path.

Add the missing 'release_firmware()' call.
Also introduce a temp variable (new_len) in order to keep the value of
'pnvm->size' after the firmware has been released.

Fixes: cdda18fb

 ("iwlwifi: pnvm: move file loading code to a separate function")
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Reviewed-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Luca Coelho <luca@coelho.fi>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/1b5d80f54c1dbf85710fd285243932943b498fe7.1630614969.git.christophe.jaillet@wanadoo.fr