drivers: scsi: mvsas: fix compiling issue by adding 'MVS_' for "enum pci_interrupt_cause"

The direct cause is IRQ_SPI is already defined as a macro in unicore32
architecture (also, blackfin and mips architectures define it). The
related error (unicore32  with allmodconfig)

    CC [M]  drivers/scsi/mvsas/mv_94xx.o
  In file included from drivers/scsi/mvsas/mv_94xx.c:27:
  drivers/scsi/mvsas/mv_94xx.h:176: error: expected identifier before numeric constant

And IRQ_SAS_A and IRQ_SAS_B are used as 'u32' (although "enum
pci_interrupt_cause" is not used directly, now).

All together, need add 'MVS_' for "enum pci_interrupt_cause".

Signed-off-by: Chen Gang <gang.chen.5i5j@gmail.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Xuetao Guan <gxt@mprc.pku.edu.cn>