staging: omap-thermal: rename bg_ptr to bgp

Use a shorter name to bandgap pointer.

Cc: Benoit <b-cousson@ti.com>
Cc: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Eduardo Valentin <eduardo.valentin@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>