dfu: simplify the dependencies of DFU_TFTP

Since CONFIG_UPDATE_COMMON always selects CONFIG_DFU_WRITE_ALT, we can
drop the latter from dependencies of CONFIG_DFU_TFTP.

Fixes: 3149e524 ("common: update: add a generic interface for FIT
       image")
Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>