pciehp: Remove unnecessary check in pciehp_ctrl.c

this was spotted by coverity (cid #819). We dereference p_slot
earlier in the function, and i found no way it could become NULL
anywhere.

Signed-off-by: Eric Sesterhenn <snakebyte@gmx.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>