[POWERPC] Fix section mismatches in udbg_adb.c

The functions are only called from __init functions.

WARNING: vmlinux.o(.text+0x45ed0): Section mismatch: reference to .init.text:.btext_find_display (between '.udbg_adb_init_early' and '.udbg_adb_init')
WARNING: vmlinux.o(.text+0x45f9c): Section mismatch: reference to .init.text:.btext_find_display (between '.udbg_adb_init' and '.udbg_adb_getc_poll')
WARNING: vmlinux.o(.text+0x46000): Section mismatch: reference to .init.text:.find_via_pmu (between '.udbg_adb_init' and '.udbg_adb_getc_poll')

Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>