clk: stm32mp1: solve type issue in stm32mp1_lse_enable and stm32mp1_clktree

Solve type issue in stm32mp1_lse_enable and stm32mp1_clktree.

This patch solves the warnings when compiling with W=1
on stm32mp1 board:

clk_stm32mp1.c: In function ‘stm32mp1_lse_enable’:
clk_stm32mp1.c:1238:15: warning: comparison of integer expressions
  of different signedness: ‘u32’ {aka ‘unsigned int’} and ‘int’
  [-Wsign-compare]
clk_stm32mp1.c:1239:13: warning: comparison of integer expressions
  of different signedness: ‘u32’ {aka ‘unsigned int’} and ‘int’
  [-Wsign-compare]

clk_stm32mp1.c: In function ‘stm32mp1_clktree’:
clk_stm32mp1.c:1814:17: warning: comparison of integer expressions
  of different signedness: ‘int’ and ‘unsigned int’
  [-Wsign-compare]

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@st.com>