fbnic: Change kconfig prompt from S390=n to !S390

In testing the recent kernel I found that the fbnic driver couldn't be
enabled on x86_64 builds. A bit of digging showed that the fbnic driver was
the only one to check for S390 to be n, all others had checked for !S390.
Since it is a boolean and not a tristate I am not sure it will be N. So
just update it to use the !S390 flag.

A quick check via "make menuconfig" verified that after making this change
there was an option to select the fbnic driver.

Fixes 0e03c643

 ("eth: fbnic: fix s390 build.")
Signed-off-by: Alexander Duyck <alexanderduyck@fb.com>
Reviewed-by: Joe Damato <jdamato@fastly.com>
Link: https://patch.msgid.link/172192698293.1903337.4255690118685300353.stgit@ahduyck-xeon-server.home.arpa
Signed-off-by: Jakub Kicinski <kuba@kernel.org>