staging: nvec: Allow TX buffers only in the upper 75% of the pool

Allow TX buffers to be allocated only in the upper 75% of the pool
to avoid a completely filled buffer preventing the driver from
processing responses. This also improves performance, as RX
allocations do not require checking buffers allocated for TX
unless there are more than 16 incoming messages -- which is
highly unlikely.

An earlier version used the lower 75% for TX messages, but
that was considered to be not that effective due to the
overlaps of RX and TX buffers mentioned above.

Signed-off-by: Julian Andres Klode <jak@jak-linux.org>
Acked-by: Marc Dietrich <marvin24@gmx.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>