drivers: i2c: i2c_mcux_lpi2c: fix base address declarations

With commit 734adf52, the MCUX LPI2C config structure no longer
contains a direct base address pointer. The base address must be
accessed via DEVICE_MMIO_NAMED_GET. Some declarations in the LPCI2C
target mode handler still used the old method of accessing the base
address, causing a build failure. Fix these accesses to use the local
declaration of the "base" variable.

Signed-off-by: Daniel DeGrasse <daniel.degrasse@nxp.com>