libflashrom.c: Use casts on enum conversions

This allows flashrom to build with GCC 10.

Change-Id: I2166cdf3681452631ef8e980face2924e9a6c81a
Signed-off-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-on: https://review.coreboot.org/c/flashrom/+/41775


Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: HAOUAS Elyes <ehaouas@noos.fr>