pppoatm: do not inline pppoatm_may_send()

The pppoatm_may_send() is quite heavy and it's called three times
in pppoatm_send() and inlining costs more than 200 bytes of code
(more than 10% of total pppoatm driver code size).

add/remove: 1/0 grow/shrink: 0/1 up/down: 132/-367 (-235)
function                                     old     new   delta
pppoatm_may_send                               -     132    +132
pppoatm_send                                 900     533    -367

Signed-off-by: Krzysztof Mazur <krzysiek@podlesie.net>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>