net: mana: Use int to check the return value of mana_gd_poll_cq()

mana_gd_poll_cq() may return -1 if an overflow error is detected (this
should never happen unless there is a bug in the driver or the hardware).

Fix the type of the variable "comp_read" by using int rather than u32.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Fixes: ca9c54d2 ("net: mana: Add a driver for Microsoft Azure Network Adapter (MANA)")
Signed-off-by: Dexuan Cui <decui@microsoft.com>
Signed-off-by: David S. Miller <davem@davemloft.net>