enic: bug fix: return notify intr credits

Return notify intr credits after notify intr from firmware.  This is
especially important for legacy PCI intr mode, where not returning
credits would cause PBA to remain asserted which would get us right
back into the ISR.

Signed-off-by: Scott Feldman <scofeldm@cisco.com>
Signed-off-by: David S. Miller <davem@davemloft.net>