lwip: Round up ms to tick conversion

LwIP expresses timeout in milliseconds which are latter converted into
ticks. Depending on the tick frequency of the system, the previous
conversion routine was returning a tick value of 0 for certain amounts
of ms.

This update take into account the remainder of division made for the
conversion. If a remainder is present, it unconditionally adds 1 tick.

This solves issues in keyword where the system request to sleep for 3ms
but sleeps for 0 ticks. This ends up in busy waiting.

Signed-off-by: Vincent Coubard <vincent.coubard@arm.com>