sis900: Allocate rx replacement buffer before rx operation

	The sis900 driver appears to have a bug in which the receive routine
passes the skbuff holding the received frame to the network stack before
refilling the buffer in the rx ring.  If a new skbuff cannot be allocated, the
driver simply leaves a hole in the rx ring, which causes the driver to stop
receiving frames and become non-recoverable without an rmmod/insmod according to
reporters.  This patch reverses that order, attempting to allocate a replacement
buffer first, and receiving the new frame only if one can be allocated.  If no
skbuff can be allocated, the current skbuf in the rx ring is recycled, dropping
the current frame, but keeping the NIC operational.

Signed-off-by: Neil Horman <nhorman@tuxdriver.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>