omap: Remove DEBUG_FS dependency for mux name checking

The check for a valid mux name should be performed regardless of whether
DEBUG_FS is enabled or not. Otherwise without DEBUG_FS, we get:

Unable to handle kernel NULL pointer dereference at virtual address 00000000
pgd = c0004000
[00000000] *pgd=00000000
Internal error: Oops: 5 [#1]
last sysfs file:
Modules linked in:
CPU: 0    Not tainted  (2.6.33-rc8 #10)
PC is at strcmp+0x18/0x40
LR is at omap_mux_init_signal+0x68/0x14c
...

This fixes the issue currently seen with boards not booting up
if DEBUG_FS is not enabled in defconfig.

Note that the earlier ifndef + ifdef now becomes simpler ifdef else:
If CONFIG_OMAP_MUX is selected, we use pin names. If it's not selected,
we only want the GPIO to mux register mapping.

Signed-off-by: Ranjith Lohithakshan <ranjithl@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>