microblaze: Remove additional setup for kernel_mode

PT_MODE stores information if kernel comes from user
or kernel space. If come from user space, PT_MODE
contains 0. If come from kernel store, PT_MODE contains
non zero value. We don't need to save value 1. I am using
r1 register which contains non zero value.
This change save one additional instruction.

Signed-off-by: Michal Simek <monstr@monstr.eu>