drm/amdgpu/pm: Account for extra separator characters in sysfs interface

drm/amdgpu/pm: Account for extra separator characters in sysfs interface

Currently, the the input to the pp_od_clk_voltage sysfs interface has
two adjacent separator characters, i.e. "\n\0", then we try to parse an
argument out of the empty string, whereas we really should just ignore
this case, and treat any number of adjacent separators as one separator
for arguments.

We do this here by simply skipping the argument parsing for all
empy-string arguments.

An example test case would be `echo 's  1 900' > pp_od_clk_voltage`
(note the two spaces after the 's'.

This also solves the issue where the written string ends with both a
newline, and a nul-terminator, '\n\0', as is the case for `echo 's 1
900' > pp_od_clk_voltage` since the recent rebase.

Got lost during code rebase/merge. No need to port this to other
branches.

Reviewed-by: Evan Quan <evan.quan@amd.com>
Signed-off-by: Matt Coffin <mcoffin13@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>