fs: fix ext4 write() conditional assignment

The Kconfig for the ext4 filesystem has a dedicated option to enable
write support documented in doc/README.ext4:

> First, to enable support for both ext4 (and, automatically, ext2 as well),
> but without selecting the corresponding commands, enable one of the following:

>  CONFIG_FS_EXT4        (for read-only)
>  CONFIG_EXT4_WRITE     (for read-write)

Currently, ext4 write is only enabled if we select  CONFIG_*CMD*_EXT4,
which is wrong.

Signed-off-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>
1 file changed