When CAN_FLEXCAN=y and M5441x is not set/enabled, there are build
errors in coldfire/device.c:

../arch/m68k/coldfire/device.c:595:26: error: 'MCFFLEXCAN_BASE0' undeclared here (not in a function); did you mean 'MCFDMA_BASE0'?
  595 |                 .start = MCFFLEXCAN_BASE0,
../arch/m68k/coldfire/device.c:596:43: error: 'MCFFLEXCAN_SIZE' undeclared here (not in a function)
  596 |                 .end = MCFFLEXCAN_BASE0 + MCFFLEXCAN_SIZE,
../arch/m68k/coldfire/device.c:600:26: error: 'MCF_IRQ_IFL0' undeclared here (not in a function); did you mean 'MCF_IRQ_I2C0'?
  600 |                 .start = MCF_IRQ_IFL0,
../arch/m68k/coldfire/device.c:605:26: error: 'MCF_IRQ_BOFF0' undeclared here (not in a function); did you mean 'MCF_IRQ_I2C0'?
  605 |                 .start = MCF_IRQ_BOFF0,
../arch/m68k/coldfire/device.c:610:26: error: 'MCF_IRQ_ERR0' undeclared here (not in a function); did you mean 'MCF_IRQ_I2C0'?
  610 |                 .start = MCF_IRQ_ERR0,

Protect the FLEXCAN code blocks by checking if MCFFLEXCAN_SIZE
is defined.

Fixes: 35a9f936 ("m68k: m5441x: add flexcan support")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Greg Ungerer <gerg@linux-m68k.org>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: linux-m68k@lists.linux-m68k.org
Cc: uclinux-dev@uclinux.org
Cc: Angelo Dureghello <angelo@kernel-space.org>
Signed-off-by: Greg Ungerer <gerg@linux-m68k.org>