efi_loader: capsule: fix SIZEOF_MISMATCH warning

CID 316361 says:
> Passing argument "count * 8UL /* sizeof (*files) */" to
> function "dlmalloc" and then casting the return value to "u16 **" is
> suspicious. In this particular case "sizeof (u16 **)" happens to be equal
> to "sizeof (u16 *)", but this is not a portable assumption.
> > 767         tmp_files = malloc(count * sizeof(*files));

The change in this patch fixes the ambiguity.

Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
Fixes: CID 316361
For readability use *tmp_files instead of tmp_files[0]
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>