When MXC_ASSERT is disabled, the portexpander_config() call is not
compiled into the bootloader which leads to the portexpander not being
setup correctly and thus neither the screen turns on nor does the button
to switch into MSC mode work correctly (bootloader always goes into MSC
mode).

Fix this by moving the portexpander_config() call outside the MXC_ASSERT
and only keep the return value check inside.  Additionally fix the
pull-up/pull-down selection in portexpander_config() not being set
correctly.

Ref #171

Signed-off-by: Rahix <rahix@rahix.de>