scsi: ufs-debugfs: fix reading the M-PHY RX attributes

DME attributes read/write commands need GenSelectorIndex argument and
according to UFS host controller specification, it should specify the
targeted M-PHY data lane.
This is the valid range of GenSelectorIndex for M-PHY attributes:
	0 to (2 * PA_MaxDataLanes - 1)

Example (Note: PA_MaxDataLanes is UniPro protocol constant set to 4):
	-----------------------------------------
	MPHY Access target |	 GenSelectorIndex
	-----------------------------------------
	TX Lane0         |          0
	TX Lane1         |          1
	RX Lane0         |          4
	RX Lane1         |          5
	-----------------------------------------

This change makes sure that correct GenSelectorIndex is passed for M-PHY
attributes while it reading via debugfs.

Change-Id: If09e4271958e4bb18d315446cef253caec75584a
Signed-off-by: Subhash Jadavani <subhashj@codeaurora.org>