USB: ehci-fsl: fix regression on mpc5121e

mpc5121e doesn't have system interface registers, accessing this
register address space cause the machine check exception and a
kernel crash:

...
Machine check in kernel mode.
Caused by (from SRR1=49030): Transfer error ack signal
Oops: Machine check, sig: 7 [#1]
MPC5121 ADS
Modules linked in:
NIP: c025fd60 LR: c0265bb4 CTR: 00000000
REGS: df82dac0 TRAP: 0200   Not tainted
(3.7.0-rc7-00641-g81e6c91)
MSR: 00049030 <EE,ME,IR,DR>  CR: 42002024  XER: 20000000
TASK = df824b70[1] 'swapper' THREAD: df82c000
GPR00: 00000000 df82db70 df824b70 df3ed0f0 00000003 00000000 00000000 00000000
GPR08: 00000020 32000000 c03550ec 20000000 22002028 00000000 c0003f5c 00000000
GPR16: 00000000 00000000 00000000 00000000 00000000 00000000 c0423898 c0450000
GPR24: 00000077 00000002 e5086180 1c000c00 e5086000 df33ec00 00000003 df34e000
NIP [c025fd60] ehci_fsl_setup_phy+0xd0/0x354
LR [c0265bb4] ehci_fsl_setup+0x220/0x284
...

Fix it by checking 'have_sysif_regs' flag before register access.

Signed-off-by: Anatolij Gustschin <agust@denx.de>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>