staging: sm750fb: Make internal functions static

This patch declares externally unavailable functions static. This fixes
the following sparse warnings:-

drivers/staging/sm750fb/ddk750_hwi2c.c:63:6: warning: symbol 'hwI2CWaitTXDone' was not declared. Should it be static?
drivers/staging/sm750fb/ddk750_hwi2c.c:93:14: warning: symbol 'hwI2CWriteData' was not declared. Should it be static?
drivers/staging/sm750fb/ddk750_hwi2c.c:160:14: warning: symbol 'hwI2CReadData' was not declared. Should it be static?

Signed-off-by: Lorenzo Stoakes <lstoakes@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>