[ARM] Fix pxamci regression

Fix:

WARNING: at arch/arm/mach-pxa/clock.c:69 clk_disable()
[<c002d7c8>] (dump_stack+0x0/0x14) from [<c00334f4>] (clk_disable+0x34/0xa0)
[<c00334c0>] (clk_disable+0x0/0xa0) from [<c028a43c>] (pxamci_set_ios+0x74/0xf0)
[<c028a3c8>] (pxamci_set_ios+0x0/0xf0) from [<c0281548>] (mmc_power_off+0x90/0x9c)
[<c02814b8>] (mmc_power_off+0x0/0x9c) from [<c0281a30>] (mmc_start_host+0x18/0x28)
[<c0281a18>] (mmc_start_host+0x0/0x28) from [<c02825a0>] (mmc_add_host+0xe8/0x104)
[<c02824b8>] (mmc_add_host+0x0/0x104) from [<c028a7d0>] (pxamci_probe+0x24c/0x2f4)
[<c028a584>] (pxamci_probe+0x0/0x2f4) from [<c01e5948>] (platform_drv_probe+0x20/0x24)
...

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>