IB/ehca: handle negative return value from ibmebus_request_irq() properly

ehca_create_eq() was assigning a signed return value to an unsiged
local variable and then checking if the variable was < 0, which meant
that errors were always ignored.  Fix this by using one variable for
signed integer return values and another for u64 hcall return values.

Bug originally found by Roel Kluin <12o3l@tiscali.nl>.

Signed-off-by: Hoang-Nam Nguyen <hnguyen@de.ibm.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>