efi: Fix ESRT refresh after Capsule update

Indicated by Coverity Scan CID 331147

The ESRT was being refreshed in situations where the UpdateCapsule
procedure failed. In that scenario:

1) the ESRT refresh was superfluous.
2) a failed ESRT refresh return code overwrites the UpdateCapsule error
return code.

This commit ensures that the ESRT is only refreshed when the
UpdateCapsule performs successfully.

CC: Heinrich Schuchardt <xypron.glpk@gmx.de>
CC: Sughosh Ganu <sughosh.ganu@linaro.org>
CC: AKASHI Takahiro <takahiro.akashi@linaro.org>
CC: Tom Rini <trini@konsulko.com>
CC: Andre Przywara <andre.przywara@arm.com>
CC: nd@arm.com

Signed-off-by: Jose Marinho <jose.marinho@arm.com>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>