V4L/DVB (6577): et61x251/: make 5 functions static

This patch makes the following needlessly global functions in
et61x251_core.c static:
- et61x251_read_reg()
- et61x251_i2c_try_read()
- et61x251_i2c_try_write()
- et61x251_i2c_read()
- et61x251_i2c_write()

Signed-off-by: Adrian Bunk <bunk@kernel.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>