media: em28xx-reg.h: Fix coding style issues

Use BIT() macros and fix one comment that is not following
the Kernel coding style.

It should be noticed that the registers bit masks should be
casted to unsigned char, as, otherwise, it would produce
warnings like:

	drivers/media/usb/em28xx/em28xx-cards.c:81:33: warning: large integer implicitly truncated to unsigned type [-Woverflow]
	  {EM2820_R08_GPIO_CTRL, 0x6d,   ~EM_GPIO_4, 10},
	                                 ^

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>