efivars: pstore: Do not check size when erasing variable

In 3.2, unlike mainline, efi_pstore_erase() calls efi_pstore_write()
with a size of 0, as the underlying EFI interface treats a size of 0
as meaning deletion.

This was not taken into account in my backport of commit d80a361d779a
'efi_pstore: Check remaining space with QueryVariableInfo() before
writing data'.  The size check should be omitted when erasing.

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>