powerpc: udbg_memcons: mark functions static

ppc64_book3e_allmodconfig has one more driver that triggeres a
few missing-prototypes warnings:

arch/powerpc/sysdev/udbg_memcons.c:44:6: error: no previous prototype for 'memcons_putc' [-Werror=missing-prototypes]
arch/powerpc/sysdev/udbg_memcons.c:57:5: error: no previous prototype for 'memcons_getc_poll' [-Werror=missing-prototypes]
arch/powerpc/sysdev/udbg_memcons.c:80:5: error: no previous prototype for 'memcons_getc' [-Werror=missing-prototypes]

Mark all these function static as there are no other users.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20240123125148.2004648-1-arnd@kernel.org