Fix TFTP buffer size

Currently shim can't boot with PXE. It fails with

    Fetching Netboot image
    Invalid signature
    Verification failed

Because bufsiz needs to be the exact file size but now the hashes are calculated on the default 4MB buffer, and thus, wrong.

&bufsiz was a typo.

"For read-file operations, if `EFI_BUFFER_TOO_SMALL` is returned, `*BufferSize` returns the size of the requested file."

So just use it.

Signed-off-by: Lingzhu Xiang <lxiang@redhat.com>