megaraid_sas : Add separate function for setting up IRQs

This patch will create separate functions for- 1) setting up IRQs for MSI-x
interrupts 2) setting up IRQs for legacy interrupts 3) freeing up IRQs.  and
enable interrupts after adapter's initialization. The reason behind
initialising adapter earlier is: by that time firmware is operational and can
send interrupts, so better to use interrupt based interface to send internal
DCMD to firmware instead of using polling method, since MFI frames' pool size
is reduced and polling method does not free up MFI frame for fusion adapters,
so sending more DCMDs with polled method may cause MFI frames's pool go out of
frames and end up failing DCMD.

Signed-off-by: Kashyap Desai <kashyap.desai@avagotech.com>
Signed-off-by: Sumit Saxena <sumit.saxena@avagotech.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: James Bottomley <JBottomley@Odin.com>