[SCSI] megaraid_sas: Use correct #define for MSI-X capability

Previously we used PCI_MSI_FLAGS to locate a register in the MSI-X
capability.  This did work because the MSI and MSI-X flags happen
to be at the same offsets, but was confusing.

PCI_MSIX_FLAGS_ENABLE is already defined in include/uapi/linux/pci_regs.h,
so no need to define it again.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Adam Radford <aradford@gmail.com>