spi: spi_qsd: Fix the register peek/poke debug feature

The spi_qsd driver allows peeking/poking registers via debugfs.

Currently a static variable is used as the user data to get to the spi
controller data structure. Unfortunately this means that the last device
to probe is always obtained.

Fix this to use a member of the spi controller data structure instead.

Change-Id: I711354941b4168f3f6ffe2d29185597bdad4da89
Signed-off-by: Girish Mahadevan <girishm@codeaurora.org>