dp8393x: Don't stop reception upon RBE interrupt assertion

dp8393x: Don't stop reception upon RBE interrupt assertion

Section 3.4.7 of the datasheet explains that,

    The RBE bit in the Interrupt Status register is set when the
    SONIC finishes using the second to last receive buffer and reads
    the last RRA descriptor. Actually, the SONIC is not truly out of
    resources, but gives the system an early warning of an impending
    out of resources condition.

RBE does not mean actual receive buffer exhaustion, and reception should
not be stopped. This is important because Linux will not check and clear
the RBE interrupt until it receives another packet. But that won't
happen if can_receive returns false. This bug causes the SONIC to become
deaf (until reset).

Fix this with a new flag to indicate actual receive buffer exhaustion.

Signed-off-by: Finn Thain <fthain@telegraphics.com.au>
Tested-by: Laurent Vivier <laurent@vivier.eu>
Signed-off-by: Jason Wang <jasowang@redhat.com>