ppp: Fix PPPIOCUNBRIDGECHAN request number

PPPIOCGL2TPSTATS already uses 54. This shouldn't be a problem in
practice, but let's keep the logical decreasing assignment scheme.

Fixes: 4cf476ce ("ppp: add PPPIOCBRIDGECHAN and PPPIOCUNBRIDGECHAN ioctls")
Signed-off-by: Guillaume Nault <gnault@redhat.com>
Link: https://lore.kernel.org/r/e3a4c355e3820331d8e1fffef8522739aae58b57.1608380117.git.gnault@redhat.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>