scripts: ipe: polgen: remove redundant close and error exit path

Currently if an fopen fails the error exit path is via code that
checks if fp is not null and closes the file, however, fp is null
so this check and close is redundant. Since the only use of the
err exit label is on the fopen check, remove it and replace the
code with a simple return of errno. Also remove variable rc since
it's no longer required.

Signed-off-by: Colin Ian King <colin.i.king@gmail.com>
Signed-off-by: Fan Wu <wufan@kernel.org>