media: cxd2880-spi: Fix a null pointer dereference on error handling path

Currently the null pointer check on dvb_spi->vcc_supply is inverted and
this leads to only null values of the dvb_spi->vcc_supply being passed
to the call of regulator_disable causing null pointer dereferences.
Fix this by only calling regulator_disable if dvb_spi->vcc_supply is
not null.

Addresses-Coverity: ("Dereference after null check")

Fixes: dcb01458 ("media: cxd2880-spi: Fix an error handling path")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Sean Young <sean@mess.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>