Fix bootm to work on powerpc again (compressed uImage)

Patch 35fc84fa [Refactor the bootm command to reduce code duplication]
breaks booting Linux (compressed uImage with fdt) on powerpc.

boot_jump_linux() mustn't be called before boot_prep_linux() and
boot_body_linux() have been called. So remove the superfluous call
to boot_jump_linux() in arch/powerpc/lib/bootm.c as its called later on
in this function.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Simon Glass <sjg@chromium.org>
Cc: Tom Rini <trini@ti.com>