efi: Fix types in EFI calls to match EFI function definitions.

EFI calls can made directly on ARM, so the function pointers
are directly invoked.  This allows types to be checked at
compile time, so here we ensure that the parameters match
the function signature. The wrappers used by x86 prevent
any type checking.
Correct the type of chunksize to be based on native
width as specified by the EFI_FILE_PROTOCOL read()
function.

Signed-off-by: Roy Franz <roy.franz@linaro.org>
Signed-off-by: Matt Fleming <matt.fleming@intel.com>