tty/powerpc: fix build break with ehv_bytechan.c on allyesconfig

The ePAPR hypervisor byte channel driver is supposed to work on all
ePAPR-compliant embedded PowerPC systems, but it had a reference to the MSR_GS
bit, which is available only on Book-E systems.

Also fix a couple integer-to-pointer typecast problems.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Timur Tabi <timur@freescale.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>