cmd: clk: test the number of argument in setfreq command

cmd: clk: test the number of argument in setfreq command

Test the number of argument in setfreq command to avoid a crash when
the command setfreq is called without argument:

  STM32MP> clk setfreq
  data abort
  pc : [<ddba3f18>]	   lr : [<ddba3f89>]
  reloc pc : [<c018ff18>]	   lr : [<c018ff89>]
  sp : dbaf45b8  ip : ddb1d859	 fp : 00000002
  r10: dbb3fd80  r9 : dbb11e90	 r8 : ddbf38cc
  r7 : ddb39725  r6 : 00000000	 r5 : 00000000  r4 : dbb3fd84
  r3 : dbb3fd84  r2 : 0000000a	 r1 : dbaf45bc  r0 : 00000011
  Flags: nzCv  IRQs off  FIQs off  Mode SVC_32 (T)
  Code: 4dd3 1062 85a3 ddbd (7803) 2b30
  Resetting CPU ...

Fixes: 7ab418fbe612 ("clk: add support for setting clk rate from cmdline")
Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Reviewed-by: Sean Anderson <seanga2@gmail.com>
Link: https://lore.kernel.org/r/20220131172131.1.I32a8f213d330dccd922f7aafc60d3d63fcbe8615@changeid