part: efi: rework the partition start and size in gpt_fill_pte

The start variable is only used inside a loop, and is never affected inside
it, so it's a purely local variable.

In the same way the partition size is accessed several times, so we can
store it in a variable.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Reviewed-by: Tom Rini <trini@konsulko.com>