staging: rtl8192u: make in r8180_93cx6.c some local functions static

Make some local functions (eprom_cs, eprom_ck_cycle, eprom_w, eprom_r
and eprom_send_bits_string) static

Fix the following sparse warnings in r8180_93cx6.c

drivers/staging/rtl8192u/r8180_93cx6.c:23:6: warning: symbol 'eprom_cs' was not declared. Should it be static?
drivers/staging/rtl8192u/r8180_93cx6.c:40:6: warning: symbol 'eprom_ck_cycle' was not declared. Should it be static?
drivers/staging/rtl8192u/r8180_93cx6.c:56:6: warning: symbol 'eprom_w' was not declared. Should it be static?
drivers/staging/rtl8192u/r8180_93cx6.c:71:7: warning: symbol 'eprom_r' was not declared. Should it be static?
drivers/staging/rtl8192u/r8180_93cx6.c:85:6: warning: symbol 'eprom_send_bits_string' was not declared. Should it be static?

Signed-off-by: Ana Rey <anarey@gmail.com>
Signed-off-by: Peter P Waskiewicz Jr <peter.p.waskiewicz.jr@intel.com>