ptp: increase maximum adjustment of virtual clocks.

Increase the maximum frequency offset of virtual clocks to 50% to enable
faster slewing corrections.

This value cannot be represented as scaled ppm when long has 32 bits,
but that is already the case for other drivers, even those that provide
the adjfine() function, i.e. 32-bit applications are expected to check
for the limit.

Signed-off-by: Miroslav Lichvar <mlichvar@redhat.com>
Acked-by: Richard Cochran <richardcochran@gmail.com>
Cc: Yangbo Lu <yangbo.lu@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>