Staging: sb105x: Remove redundant null check

This patch fixes the following Sparse information in sb_pci_mp.c-

~/git/kernels/linux/drivers/staging/sb105x/sb_pci_mp.c:1813
mp_unregister_driver() info: redundant null check on drv->state calling kfree()

Signed-off-by: Rashika Kheria <rashika.kheria@gmail.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>