qfw: Fix segfault from uninitialized variables in sandbox

qfw: Fix segfault from uninitialized variables in sandbox

There are cases where qfw_read_entry() does not set the output parameter
passed by address. This occurs with qfw_sandbox_read_entry_dma, which
leaves the size variables uninitialized and causes a segfault when running
bootflow scan in U-Boot sandbox.

$ ./u-boot
...
U-Boot 2026.01-rc1-00199-gc2637036b8f0 (Nov 04 2025 - 10:32:21 +0100)
...
Hit any key to stop autoboot: 0
=> bootflow scan
     efi_var_to_file() Cannot persist EFI variables without system partition
   efi_tcg2_register() Missing TPMv2 device for EFI_TCG_PROTOCOL
    efi_rng_register() Missing RNG device for EFI_RNG_PROTOCOL
scanning bus for devices...
[3]    1015761 segmentation fault (core dumped)  ./u-boot

Initalize all these variables to 0 to fix this issue.

Signed-off-by: Kory Maincent (TI.com) <kory.maincent@bootlin.com>