cpu/stm32_common: Do not use USEMODULE to select i2c implem

`cpu/stm32_common/Makefile.dep` was never included by the global
`Makefile.dep`, so declaring this `USEMODULE +=` here was never shown to
the build system and never exported as `MODULE_PERIPH_I2C_X` variables.

In fact, `USEMODULE` was only used to trigger the `periph.mk`/`SUBMODULES`
mechanism to add matching source files names to `SRC` and so select
the implementation for each CPU type.

It is replaced by just explicitly selecting the right source file.

cpu/stm32_common/Makefile.dep

-ifneq (,$(filter periph_i2c,$(USEMODULE)))
-  ifneq (,$(filter $(CPU),stm32f0 stm32f3 stm32f7 stm32l0 stm32l4))
-    USEMODULE += periph_i2c_1
-  else # stm32f1/f2/f4/l1
-    USEMODULE += periph_i2c_2
-  endif
-endif

cpu/stm32_common/periph/Makefile

 MODULE = stm32_common_periph
 
-include $(RIOTCPU)/stm32_common/Makefile.dep
+# Select the specific implementation for `periph_i2c`
+ifneq (,$(filter periph_i2c,$(USEMODULE)))
+  ifneq (,$(filter $(CPU),stm32f0 stm32f3 stm32f7 stm32l0 stm32l4))
+    SRC += i2c_1.c
+  else # stm32f1/f2/f4/l1
+    SRC += i2c_2.c
+  endif
+endif
 
 include $(RIOTMAKE)/periph.mk