mtip32xx: dynamically allocate buffer in debugfs functions

Dynamically allocate buf to prevent warnings:

drivers/block/mtip32xx/mtip32xx.c: In function ‘mtip_hw_read_device_status’:
drivers/block/mtip32xx/mtip32xx.c:2823: warning: the frame size of 1056 bytes is larger than 1024 bytes
drivers/block/mtip32xx/mtip32xx.c: In function ‘mtip_hw_read_registers’:
drivers/block/mtip32xx/mtip32xx.c:2894: warning: the frame size of 1056 bytes is larger than 1024 bytes
drivers/block/mtip32xx/mtip32xx.c: In function ‘mtip_hw_read_flags’:
drivers/block/mtip32xx/mtip32xx.c:2917: warning: the frame size of 1056 bytes is larger than 1024 bytes

Signed-off-by: David Milburn <dmilburn@redhat.com>
Acked-by: Asai Thambi S P <asamymuthupa@micron.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>