mmc: rk29: fix NULL dereference in rk29_sdmmc_set_frq

mmc: rk29: fix NULL dereference in rk29_sdmmc_set_frq

fix this bug:
Unable to handle kernel NULL pointer dereference at virtual address 000000d0
pgd = c0404000
[000000d0] *pgd=00000000
Internal error: Oops: 5 [#1] PREEMPT
CPU: 0    Not tainted  (3.0.8+ #5)
PC is at rk29_sdmmc_set_frq+0x28/0x6c
LR is at rk29_sdmmc_set_frq+0x10/0x6c
pc : [<c06c664c>]    lr : [<c06c6634>]    psr: 80000093
sp : d53c9e28  ip : ffff8b5d  fp : c0aae32c
r10: 00000000  r9 : d53a24d0  r8 : ffff8b5c
r7 : 60000013  r6 : 00002041  r5 : d53c9ed8  r4 : d53bc280
r3 : 00000000  r2 : 00000000  r1 : d53c9ed8  r0 : d53bc000
[<c06c664c>] (rk29_sdmmc_set_frq+0x28/0x6c) from [<c06c66e8>] (rk29_sdmmc_start_command+0x58/0x120)
[<c06c66e8>] (rk29_sdmmc_start_command+0x58/0x120) from [<c06c6d40>] (rk29_sdmmc_request+0x590/0x704)
[<c06c6d40>] (rk29_sdmmc_request+0x590/0x704) from [<c06bac2c>] (mmc_wait_for_req+0x110/0x214)
[<c06bac2c>] (mmc_wait_for_req+0x110/0x214) from [<c06bad94>] (mmc_wait_for_cmd+0x64/0x74)
[<c06bad94>] (mmc_wait_for_cmd+0x64/0x74) from [<c06bd71c>] (mmc_send_op_cond+0x74/0xe0)
[<c06bd71c>] (mmc_send_op_cond+0x74/0xe0) from [<c06bca78>] (mmc_attach_mmc+0x40/0x1f0)
[<c06bca78>] (mmc_attach_mmc+0x40/0x1f0) from [<c06baa98>] (mmc_rescan+0x344/0x3c8)
[<c06baa98>] (mmc_rescan+0x344/0x3c8) from [<c04680c4>] (process_one_work+0x258/0x3ac)
[<c04680c4>] (process_one_work+0x258/0x3ac) from [<c046aa74>] (worker_thread+0x238/0x440)
[<c046aa74>] (worker_thread+0x238/0x440) from [<c046e4c0>] (kthread+0x80/0x88)
[<c046e4c0>] (kthread+0x80/0x88) from [<c04331d0>] (kernel_thread_exit+0x0/0x8)