fsi: cleanup extern usage in function definition

Smatch reports these issues
fsi-core.c:395:12: warning: function 'fsi_slave_claim_range'
  with external linkage has definition
fsi-core.c:409:13: warning: function 'fsi_slave_release_range'
  with external linkage has definition

The storage-class-specifier extern is not needed in a
definition, so remove it.

Signed-off-by: Tom Rix <trix@redhat.com>
Link: https://lore.kernel.org/r/20220403140937.3833578-1-trix@redhat.com


Signed-off-by: Joel Stanley <joel@jms.id.au>