clk: lpc32xx: fix compilation warning

Remove the following false positives compilation warning:
- drivers/clk/nxp/clk-lpc32xx.c: In function 'lpc32xx_clk_register':
  warning: 'hw' may be used uninitialized in this function [-Wmaybe-uninitialized]
- drivers/clk/nxp/clk-lpc32xx.c: In function 'clk_hclk_pll_round_rate':
  warning: 'p' may be used uninitialized in this function [-Wmaybe-uninitialized]
  warning: 'n' may be used uninitialized in this function [-Wmaybe-uninitialized]
  warning: 'm' may be used uninitialized in this function [-Wmaybe-uninitialized]

Tested using gcc version 4.7.3.

Signed-off-by: Sylvain Lemieux <slemieux@tycoint.com>
[sboyd@codeaurora.org: Drop assignment of hw to NULL as return
silences it and is less likely to lead to hiding problems later]
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>