drivers: i2c: i2c_mcux_lpi2c: fix unused variable warning

With commit 1d7476af, it fixed a build error with config
structure no longer having a base address field but left
the local variable defined. This resulted in a build warning
of an unused variable 'config' in two places.

Signed-off-by: David Leach <david.leach@nxp.com>