media: atomisp: fix inverted error check for ia_css_mipi_is_source_port_valid()

The function ia_css_mipi_is_source_port_valid() returns true if the port
is valid. So, we can't use the existing err variable as is.

To fix this issue while reusing that variable, invert the return value
when assigning it to the variable.

Fixes: 3c0538fb ("media: atomisp: get rid of most checks for ISP2401 version")
Signed-off-by: Tsuchiya Yuto <kitakar@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>