Revert "kernel: generic: fix fonfxc uimage parser"

Revert "kernel: generic: fix fonfxc uimage parser"

This reverts commit e92a14709d37f64dcba8c81fa51e61e5f10f439a.

mtdsplit_uimage_parse_fonfxc() gets called in two situations:
1) It was /requested/ from DT using "fonfxc,uimage" compatible string
2) It was called by parsing code after finding "firmware"
   (MTD_SPLIT_FIRMWARE_NAME) due to the parser's type

Code added in the /fix/ commit basically just disabled the second case.
If that's the real goal it could be achieved by simply dropping type
MTD_PARSER_TYPE_FIRMWARE. It may however require another solution as
it's possible that some non-DT target actually needs fonfxc uImage
parsing.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>